Here's a list of shops making factory cast, resin cast, or 3d print parts and accessories that might be useful to customizers.  

I'll admit, it's a biased list; I only collect and make 3.75 figures so this list is focused on that. Some of these shops also make stuff for other scales, but if a shop doesn't offer 3.75 I won't be including it here. 

Also, I know there are a bunch of guys making and selling 3d models for people to print at home. I'm not including those here either; these shops sell physical products. 

**Updated to alphabetize**

AegisBazaar - Etsy Link. I have not yet used this shop.

Angry Raccoon Minis - I have used this shop a few times, great quality, will use again soon!

Avacs Lab - I have not yet used this shop.

Clanker Commissions - I've not yet used this shop, the site lists everything in 1:6 and 1:12, but he says to contact him and everything he makes can be done 1:18th.

Craniac Studios - I've not yet used this shop, only recently heard about them.

Customizer Corner - Searchable data clearinghouse allowing you to see results from some (but not all) of these producers in one place. A good place to start if you're looking for some specific part.

Demon Creations - I have not yet used this shop.

Hole in the Ground - I have used this shop a few times, great quality, will use again soon!

Kamino 3d Facility - I have not yet used this shop.

Krazy D's Kustoms - I have used this shop once, great quality, would use again.

Mistress Mouse - Etsy link. I have not yet used this shop, only recently heard about it.

Outer Rim Traders - I have used this shop a few times, great quality, will use again soon!

Planetary Dogs - I've not yet used this shop, only recently heard about them.

Production Platform 3- I have not yet used this shop.

Rebel Kid Creations - I have not yet used this shop, but they're reputation in the community is unparalleled; you are safe ordering from them. 

Redflare Studio - And Redflare on Etsy - I have used this shop a few times, great quality, will use again soon!

Replacement Sabers - I've not yet used this shop, only recently heard about them.

SandCrawler Customs - Facebook link, but they take orders on Instagram too. I have not yet used this shop.

Scoundrels Stock - I have not yet used this shop.

Seinar Custom Shop - I have not yet used this shop.

Spartan 118 - I have not yet used this shop.

Starkiller 3d Customs - I don't think they have a digital storefront, but they take orders on Facebook. I have not yet used this shop.

Star Wars Geek Customs- eBay Link. I have not yet used this shop.

Syndicate Customs - Based in Australia, but will ship overseas. I have not yet used this shop.

Trash Compactor Customs - I have used this one a few times, great quality, will use again soon!

Uzay's Customs - I have not yet used this shop.

Vortious Customs - I have not yet used this shop.

Watto's Scrapyard - eBay Link, but they are also taking orders on Facebook and Instagram.

Other: 

Marauders Inc - This is THE place for 3.75 military figures and accessories. Many of their accessories can be used for Star Wars customs. 

Vitruvian Armory - This shop sells Boss Fight, Dime Novel, and other figure lines, but they sell individual parts. Great place to find interesting parts and accessories piecemeal. 

Empire Toy Works - Ebay link. Well known world builder selling amazing buildings, speeders, and other dio accessories.

Stan Solo Creations - Not really a custom parts shop, they sell factory produced vintage style customs and repro, but they're really good. 

Beggars Canyon - Not really a custom parts shop, they offer vintage style custom recard kits and services.

Asis Film Models - Not really a custom parts shop, they sell finished product customs, but they're really good. 

Bad Mother Customs - Makes decals for figures and vehicles.

DioWarriors Store - Not focused on Star Wars, but they have some really interesting dio items.

Dio Display - Not focused on Star Wars, but they have some really interesting dio items.

There are a bunch of sellers on Etsy & eBay that I have not gotten to, if you know of one that should be included here please contact me.