Keycaps are mostly done thru the manufacturers GMK and Signature Plastics (unless you are talking about artisans, in which case no clue). Chances are if you email either of them they will be able to tell you what you need to do to design and make it real. To sell them, there are people in the community who will sell you renders if you don't know how to make them yourself.
You mentioned other keyboard components, so:
laser cut stuff - ponoko, laserboost
machined stuff - most people won't say their manufacturer
PCBs - JLCPCB, PCBWay, most PCB manufacturers do it fine if you don't have any particular requirements
Aliexpress and Alibaba for all sorts of manufacturers and parts