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.
Good Luck!
Josh Merrow
http://jmerrowstudio.com
... when fits of creativity run strong, more than one programmer or writer has been known to abandon the desktop for the more spacious floor. -- Fred Brooks