This is maybe a longshot, but...
I bought a toy tank at a thrift store the other day for $5.
The circuitry inside it was fried chicken, but that was crap anyway. It had two motors with gear reduction on each. It's an inexpensive plastic gear box, but it looks just like several of the ones I see for sale on robotic sites.
The treads and motors and gearboxes work fine, which is what I wanted anyway. Removing a few screws allows the top to pop off. Tadaa! Instant and inexpensive track robot chassis. Pretty good sized rectangular space for electronics and batteries between the tracks.
OK I'm done gloating. But the point is, you may very well find some suitable robot chassis and other parts at a local thrift store. Now that I'm into robots, I never skip the toy aisle at regular stores either, but thrift stores are great places to rummage through for robot parts and ideas. Give that a shot.
I plan to dream up some kind of H-bridge driver of my own to drive this thing. You might google and do a little reading about H-bridge drivers. It's a pretty simple and cheap way to drive motors. Here's a couple prett good links about them (Link)