Here's a list of vendors I typically visit for parts and merch. They are listed in alphabetical order.
(Not affiliated or sponsored by any of them unless specifically noted!)
Rails and Internal Reinforcements
Aves Rails
One of the rail vendors that ooze value- the Aves name is known for their bang-for-buck build-packs, and cost effective offerings for the most popular and common projects.
Visit them at avesrails.com
Riptide Rails
Known for their finely polished rails and bespoke parts for printed projects, Riptide Rails is a Florida-based mom-and-pop machine shop that makes popular parts and uncommon ones too! They make various parts in stainless steel if you're looking for durability or finish for a project of yours.
Visit them at riptiderails.com
Part Kits
Lone Wolf Distributors
These guys got into the Glock-parts game in the early days of the DIY 80%-builds. They have every odds and ends under the sun when it comes to Glock, for not outrageous prices. (My first Poly80 was made with Lone Wolf lower parts!)
Visit them at lonewoftdist.com
(This is an affiliate link, just a heads up!)
MAF Corp.
MAF Corp. is known for their excellent assortment of... wind chimes? Well, they also have amazing and humorous merch in the form of tees, stickers, and a "Magic ATF Ball" for all your questions.
Visit them at maf-arms.com
RK Spookware
RKS is best known for packing "convenience kits". Rather than picking out all the conventional commercial-off-the-shelf hardware parts by yourself, just buy a small pack from RK and you won't have to fuss about buying a whole lot of bulk just for one or two builds.
Visit them at rkspookware.com
(This is an affiliate link, just a heads up!)
Palmetto State Armory
Home of the budget-AR part kit. PSA offers great deals on contemporary and casual DIY kits for a not-outrageous price. Popular among printers on a lower receiver print like IvanTheTroll's UBAR kit!
Visit them at palmettostatearmory.com
(This is an affiliate link, just a heads up!)
Magazines
GunMag Warehouse
I always check GMW first before I hit an auction site, especially for weird and obscure magazines. GMW seems to carry it all, for new and old guns. They also run magazine combo packs for the most popular firearms. I've ordered everything from modern Glock mags, to specialty Ruger 10/22 mags, down to ones for legacy weapons like the Hi-Power. As a plus, they have a no-fuss return policy too if you ordered more than you need like I did!
Also, they have humorous magazine-themed patches.
Visit them at gunmagwarehouse.com
(This is an affiliate link, just a heads up!)