Any small manufacturing shop with a 3 axis CNC mill will do prototype work. You bring the CAD drawing to them, they convert it into a format their mill will read, and they'll cut it out of whatever you specify. Make sure to get a non-disclosure/non-compete signed before turning over your CAD drawings to protect your idea from going any further than you want it to. Also, if you've got any major changes to what's already being done, then at least apply for a patent; that establishes your intent and could protect you later on from a major manufacturer copying it. Actually getting a patent through runs $10-15 grand, but applying is only a few hundred and some leg work.
Bearings are different sizes based on manufacturer. Make your's whatever you want that works with your design that you can get from a reliable source in sufficient quantity whenever you want. Basically, find your bearing first then design it into the cam.
Good luck!
