I've made several kinetic sculptures using a mixture of bicycle and motorcycle parts. The best heavy duty gearing I found was in printing presses. Nice pillow blocks, shafts, gears of all flavors.
If you're near SF there's a place in the East Bay (Hayward?) called Alco that has a huge pile of machinery that they're not supposed to let you climb around in, but if you're nice...Bring tools.
Washing machines and dryers have nice belt drives. Washing machine motors are usually dual speed (slow/fast). The slow speed is helpful for sculpture, often. Lots of pumps too if you want to move liquids around.
Shaft-drive motorcycle parts are nice, though these can be expensive even when junked.
As long as we're going to reinvent the wheel again, we might as well try making it round this time. - Mike Dennison