A Canadian site specializing in custom-made plates from various materials for specific keyboard models (from a fairly long but limited list).
KPRepublic
A popular Chinese website (they also have a store on AliExpress) with a variety of products
Other Shops to Check Out
r/MechanicalKeyboards
The Reddit channel dedicated to mechanical keyboards. It features everything from community posts and intricate builds to Group Buy updates and giveaways from manufacturers.
Nouvolo
A Hong Kong-based website offering a selection of Chinese keyboards (including relatively well-known brands like Weikav), alongside keycaps, macro pads, and accessories. The site also features PC cases and cooling solutions.
DIYkeycap
A Chinese site with a broad selection of inexpensive keycaps (mostly clones of popular premium sets), and the option to design and customize your own keycap sets.
ProtoTypist
A long-standing UK website offering a wide range of GMK keycaps, PBTfans, and others, various custom keyboards, group buys, and accessories.