*Amazon is a good place to buy supplies (if you know what you need). It is NOT a good place to buy instruments.*
Musical instruments: It is a good idea to always try before you buy with instruments. You should go to a brick and mortar store or where you can look at, touch, play, and hear instruments, mouthpieces, bows, etc. Ideally, you should seek out a store where you can talk to someone who is experienced with the type of instrument you're looking for and has a repair shop you can bring your instrument to for specialized maintenance and repair.
The next best thing is purchasing a reputable brand used instrument from an individual.
Used instruments are a totally reasonable and economical option for band students and parents. Up until the last 25 years, all band instruments were built to last and stand up well over time with care and maintenance. Steve Weidner, retired Sabetha band director, sells used instruments. You can contact him at 785-285-0041. Mrs. Topp maintains a list of local people selling their used instruments that you can view here at any time. Ask friends, family, neighbors, consult local swap & talk pages, and look at garage sales. Some tried and true brands to look for are Yamaha, Jupiter, Selmer, Bundy, Conn, Bach, Gemeinhardt, Armstrong, Buffet, Holton, Benge, Getzen, Reynolds, and more. There are other smaller manufacturers out there that are very good, too. Do your homework before you buy. Feel free to contact Mrs. Topp if you're not sure about an instrument.
IMPORTANT: Please do not purchase instruments from Amazon, unless there is absolutely no alternative. There are many knockoff brands posing as legitimate instrument companies on Amazon, and their instruments do not fare well over time (especially woodwind instruments). In Mrs. Topp's experience, they work well for a year or so, then small (but frustrating) problems begin to creep in. Mrs. Topp deals with many fine-tuning issues these instruments have on a daily basis. These instruments are often not built to industry standards and cannot be repaired or have parts replaced as a result. If you purchase an instrument from Amazon, prepare to buy a replacement for it in the near future. Examples of these brands: Mendini, Ammoon, Cecilio, Kaizer, Eastar, Eastrock, Jean Paul, Lyxjam, Lazarro, etc. (these only exist on Amazon). Yamaha IS a reputable brand, and you can find Yamaha instruments on Amazon, but there are Yamaha forgeries out there as well. If the price for a new instrument is too good to be true, it probably is. Expect to pay upwards of $600 for a new instrument of a reputable brand.
Supplies: If you know what you're looking for, purchasing supplies online is safe to do. Supplies can be purchased much cheaper online than in person in most cases. A large variety of reeds, neck straps, valve oil, slide grease, etc. can be found on Amazon. If you're not sure what to purchase, please consult the recommended supplies on your instrument's Instruments and Supplies page.
A good website to purchase from that specializes in music supplies is Woodwind Brasswind. Shopping on WWBW's website will weed out most of the riffraff you'd want to avoid on Amazon. Often you can get free shipping, and it's usually very quick because they have a warehouse in Kansas City.
Sheet music: Unless you already know what you need, purchasing music online can be tricky. Some websites let you preview music now, but there are many things you can't view before purchasing. Start by looking at brick and mortar stores and asking your teachers for recommendations.