I have motors, gears, belts and power sources scavenged from electric things that have died (vacuums, blenders, etc.). Rip them apart and throw away the stuff I don't want.
It may sound a little low rent but you could drive around your neighborhood the evenings before trash days and snag some dead things for parts. You'll just have to beat the flea market scavengers to the punch.
Other than that, a gallery sounds like fun but I don't have anything low power enough to not worry about ricochets or destroy the gallery.