pic: Pneumatic

[K.Porter]

Actually, chain slippage was one problem that I didn't have with the chains.
If you notice the front axle's connection to the frame in that picture, I've milled some simple slots for the bolts as a basic chain tensioning system. I just pulled the chain taught with the whole axle assembly before tightening the bolts. That tension kept everything pretty well in place.
One problem I did have with the chain, though, was that the added tension would attempt to pull the wheels slightly out of line with each other (there's a little bit of slop between the wheels and axle, allowing that movement) Occasionally this will get to the point where the chain will jam slightly on the wheel sprockets, but it's a minor problem.

As for the air motors, I found them in the back room of my school's technology classroom. The only information printed on them is "Gast Mfg. Corp." but looking at their website, the exact model seems to be out of production. As for cost, I really have no idea. Gast doesn't even list anything on their website for current models...