Currently there are a few known keycap vendors.
Signature Plastics / Keycaps Direct is a major supplier of ABS doubleshot keys for Geek Hack group buys. They also produce other ABS keycaps as well as dye-subliminated PBT keycaps. SP keys are known for their relatively thin walls and front-facing sprue marks, so those looking for the highest quality keys may want to look elsewhere.
QWERkeys is a small outfit based in the UK that produces ABS keys. They provide in-filled legends as well as unique three-colored keys. They take custom orders.
IMSTO is a member on Geek Hack who has connections to Chinese plastic molding services. He currently sells 104-key sets of PBT keys using a standard ANSI layout. They are available in multiple colors and legends. With the right MOQ you can order keys through him but it can be rather high depending on the scope of the order.