I think it safe to assume that companies with their own costume department already know how to make and acquire costumes, so I am aiming these instructions at those smaller troupes and amateur performances who are on their own for clothing a cast.
In the old illustrations of commedia dell'arte performances, even if the stage is nothing but a bare platform without so much as a backdrop, the characters are always fully clothed in splendid costumes: they never "cheap out" and put Harlequin in street clothes. Indeed a character like Harlequin or Pierrot or Capitano is far more recognizable by his distinctive costume than by any mask.
The lesson to be learned from this is: costumes matter.
Dressing your cast well in appealing clothes is an essential investment that pays dividends in the world of low-budget stageplays. It enhances the visual experience, promotes professionalism, and fosters a stronger connection between the audience and the performance. By prioritizing costumes, you not only impress the audience but also play a crucial role in shaping a positive perception of theater. Theater is a visual art form, and costumes play a vital role in creating a captivating visual experience for the audience. Your efforts in creating an appealing presentation will undoubtedly elevate your stageplay to new heights.
The characters page describes the dress associated with the individual vecchi and zanni, and the innamorati page generally describes the clothes of the lovers. Some shows have other characters like taverners, guardsmen and musicians who also need to be outfitted. Since commedia dell'arte is usually in historical or quasi-historical settings, and the zanni and vecchi are marked out by their bizarre "clown" costumes, it's unlikely your actors have appropriate outfits just laying around their house unless they've done this type of show before.
THE FIRST THING YOU SHOULD DO IS CHOOSE THE ERA YOUR SHOW IS SET IN. You don't necessarily have to keep historically correct from there, but comprehending an approximate style is going to make it easier to choose clothes that make a unified look to the show. Even Disney fairytales, which are far from historically accurate, have an approximate place and era that the imagery is meant to evoke. Commedia dell'arte had its heyday in the 16th, 17th and 18th centuries in Western Europe, so it is usual to choose these settings for shows.
Theatrical costumes are not about fine details, since the audience is usually positioned far away; odds are they can't really see if you used a zipper to close a dress, or if the material is synthetic, or if the clothes are a bit threadbare, or if the "embroidery" is painted on and the "ribbons" are made of crepe paper. What does matter is the silhouette, and how it all looks from about 10 feet back.
I'm assuming here you're having to work primarily with thrift shop and discount store finds, or drawing from a small selection of theatre costumes available, and haven't the budget for deep historical accuracy. My suggestion here is, no matter what pieces you find, aim at matching the silhouettes. When in doubt for a character's social standing, aim at the rich person's silhouette -- the poor always liked to imitate the rich if they were able to, and audiences are likely to be more familiar with period images that depict wealthier people.
I find that, for whatever reason, the seventies of any century seem to have fashion that is associated with "the look" for the whole century, even if clothes changed a lot over the decades. So that's the model used for the following silhouette guides.
Both rich and poor men often wore waist-length capes (not shown) as a fashion accessory. Wealthy women wore long veils, and the poor women are usually shown with cloth snoods or scarves close to the head, covering their hair. The bulk around the neck of the wealthy people was achieved by either shirts with tall, gathered, frilly collars, or by attaching a separate stiffened ruff collar around the neck. The poor tend towards a "peasant look" that is very typical of European and Latin American folk dress.
Upper class men in this era had taken to wearing long, full, curled wigs (not shown.) Details are hard to see from the silhouette, but they usually wore a long cravat, sometimes with a bow at the throat, and their long coat or jerkin was closed over full petticoat breeches. Lots of ribbons, lace and frills were attached to the clothing to decorate sleeves. The poor men's outfits are beginning to resemble the kind of folk dress we're used to associating with Italy. Upper class women often wore skirts that split in the front to expose the petticoat, and the flaps of the overskirt drawn back into a bustle and train; though sometimes dresses were simply made with a separate frill at the hips to get the silhouette. Upper class women didn't usually wear veils, but when they did they were of ornate lace draped to frame the face. The poor women's clothing tried to emulate the silhouette of the upper class, sometimes wrapping an apron around the hips to imitate the effect of the bustle. Lower class women were more likely to wear a cloth snood or long scarf on their head.
In the 18th century the silhouette between rich and poor gets less distinctive, but the lower class men tend to have shorter coats almost like modern button-down shirts, and their hats tend to be wider in the country. The gentlemen wear frock coats with waistcoats of about hip length, and white cravats at the neck. The hat is tricorn style worn with wigs (not shown.) Breeches come to about calf length or a bit higher. Upper class women tend to have a lot more lace and frills on their gowns but in this era it was commented that servant girls dressed too much like the mistresses, leading to the development of "maids uniforms" in houses. Lower class women might wear scarves or mob caps. Upper class women in the 1770s were wearing very tall hairdos (not shown) that made caps or scarves difficult to use, but they sometimes donned frilly hoods instead. The lady's hairdos were not usually wigs; Fanny Burney's Evelina describes the hairdressing as one's natural hair heavily teased and padded with a "cushion" (probably made of something like wool or false hair); other sources acknowledge clip-in curls that were stuck into the hair on little bits of wire.
Occasionally you get a costumer who thinks anything with a lace-up front is a corset -- but most people use the term to mean the stiff undergarment known for pulling in the waist. Contrary to the wisdom of 19th century medical men who thought masturbation made you blind and that cigarettes cured asthma, corsets are not inherently unhealthy to wear. Idealized images in artwork show women with much smaller waists than was really typical, and the flourishing of opera singing during this period argues against the notion that one couldn't breathe in a corset.
That said, even bargain basement corsets are often about $100 each. The main thing a corset did to make one look slim was it prevented the wearer from slouching. You can buy very inexpensive posture braces, often for under $10, which will approximate the appearance. Maggie Smith reportedly faked her corset for Downton Abbey by just sitting up very straight, but it might be unreasonable to expect a stage actress to maintain this position unaided for a whole performance.
If you have a costume that won't need to be used more than a few times, cardboard and duct tape can be used to make corsets from period-correct patterns that will provide the proper stiffening and silhouette.
I advise not to skimp on these. They are to inamoratas what masks are to zanni. If Antonio Fava had played inamoratas, he'd have noticed this. The big fluffy skirts can make or break an outfit visually, they definitely impact the character's movement, and their presence or absence can well be perceived by the audience. Even putting an inamorata in nothing but underwear looks passable if a big crinoline is part of it. Make sure to get something in the correct shape for your era. Farthingales and panniers look best with many layers of gathered/frilly fabric covering them to conceal the hard outlies of the boning. Cloth crinolines are often cheaper, being made without boning, but can't stand up to very heavy fabrics quite as nicely. If neither option is afforable, hip bolsters are cheap and easy to make, being crescent shaped pillows that tie around the hips. Instructions can be found here: https://www.farthingalescorsetmakingsupplies.com/en-us/blogs/tips-tricks/instructions-for-making-a-bum-roll
True ruff collars can be made, or bought from costumers -- however they are on the delicate side and don't take a beating too well. They're made from fabric that's been heavily starched and ironed into little pleated loops. Sometimes they're faked by using wired fabric or even made from paper or cardboard, which on stage looks good enough. What I term "clown collars" are simplified versions of these that are normally not starched and are only gathered instead of carefully pleated. They were popular on commedia characters after ruffs went out of fashion in the mid-17th century. Sticking one on a zanni can really make the outfit -- a guy in a black shirt and black pants is suddenly Scaramouch with a clown collar. Guy in a white shirt and pants is now Pierrot. It's amazing!
If they only need to be good enough for stage, Halloween costume and party supply stores often have inexpensive options made from foam or plastic. Thrift shops often have options as well. Don't get hung up on whether something is considered by modern standards to be a man's hat or a woman's hat -- give the actors whatever is the right shape.
Tricorn hats are ubiquitous in 18th century settings and remained traditional for many commedia characters into the 19th and 20th century. They are made from a wide brimmed hat that has the brim turned up and folded into a triangular shape. Traditionally they are made by stiffening a cloth hat into place with shellac, but for stage, stitching or gluing the brim into place will work.
It is possible to make hats from felt, sew them together, and then using a stiffener (traditionally shellac) to set them. This allows for many of the unusual medieval and renaissance shapes to be easily and cheaply crafted. Slightly more expensive but less time consuming is the use of AVA foam to make custom hats.
A length of wide ribbon, tied as a sash, is usually cheaper than a belt, unless the actor can provide his or her own.
This is probably the biggest nightmare for a budget costumer. Even if you luck out and find $10 pairs for everyone, shoeing a cast of 10 is going to run $100. You're best saving it for characters whose shoes are visible -- women in long skirts usually won't have very noticeable shoes and can fake it out with any plain, dark colored shoe they have at home. Zanni are virtually always shown in shoes, but you might be able to get away with having them barefoot (especially if it's an acrobatic zanni who will be doing tricks).
Commedia characters are usually shown in ankle-length shoes, or in slippers of the type called clogs or mules. Knee-high or calf-high boots were a 17th century fad that innamoratos and capitanos might wear, and in the 18th century, military or "outdoorsy" men sometimes wore boots. You can find or make leather or faux-leather "covers" for shoes that make them resemble tall boots, although if you're buying them they're often just as expensive as a second-hand pair of boots that will look much better.
When buying shoes, don't get hung up on whether something is regarded as a man's shoe or woman's shoe, provided it fits the actor comfortably.
Plead with your actors that if they have any plain (no visible logos or decorations or modern shoelaces) shoes, to wear those, and you might be able to luck out with everyone having a set of suitable footwear of their own.
The final option is if you have access to a costume rental shop, they'll often have different styles of shoes in all sizes for a much lower price than having to buy brand new ones.
Wigs were a standard part of well-dressed men's fashion from the later 17th century all through the 18th century. Women sometimes wore full wigs as well, but more commonly they used only supplementary hair switches or "rats" (cushions made from hair, sometimes just whatever the women saved from their own combs).
Modern day hairstyles are often dissimilar to historical fashions, making wigs into something of interest for creating period-appropriate looks. Technology for synthetic hair has made wigs far cheaper than they once were. Short haired actors can sometimes use a clip-on hairpiece to fake out longer hair styled in a queue or bun. These hairpieces are often available for around $20. The price of a full commercial synthetic costume wig is often not too much more, although they can be clunky in appearance and suffer from issues like tangling easily (if using them for stage, I suggest sewing an invisible hairnet over them to hold them in place, or spraying them with super-hold hairspray when fresh out of the box to discourage tangles.) The sky's the limit for the cost of custom made or human hair wigs, especially if they use long hair.
Shortcuts for common costume wigs:
Men: For 16th and early 17th century, look for so-called "medieval wigs." For men's hair ca. 1660-1745 look for "judge wigs" or "captain hook wigs." For the rest of the 18th century seek "colonial wigs." For 18th century looks you can also get any long haired wig and arrange it into a queue in the back.
Women: If you just need a simple updo wig (suitable for most eras) look for "gibson girl wigs" or "geisha wigs." Very tall updos for women were really only in fashion for a short time between 1770-1790; for these search for "marie antoinette wigs" and "beehive wigs." You can sometimes evade wigs altogether by using veils or hats to cover the women's hair.
A note on "powdered wigs"
The 18th century is famed for its white wigs. In fact, these wigs were not usually white in color, but were regular hair colors lightened with use of white (or sometimes tinted) powder dusted on top. This trend started to fall increasingly out of fashion from the 1770s on. Since most costume wigs are made with synthetic hair which won't hold powder well, a substitute that I have found works well is to spray the wig with white fabric paint such as Tulip Color Shot. This stays put but also remains somewhat flexible, similar to hairspray. It also dulls the shine like a powder.