Having been the mentor (gearhead) who was responsible for the drivetrain for about 5 years, I've done a lot of searching around for (inexpensive) parts and mass-produced stuff that can be adapted to the task at hand. Besides, I'm looking for some cheap gear-reduction stuff to make a can crusher some day
Surplus Center has good prices on chain sprockets, though I don't know about the quality. They also have some good prices on shaft collars and flex couplers.
This could be very useful for some mechanism on a robot:
http://www.surpluscenter.com/Item.as...&i tem=1-2948