My recomendation would be B&Q's cheapest own brand 9.6v drill motors. (about £10 a drill i think). Then using the Robo Challenge drill motor mounts and wheels will give you a drive system ready to work. If you've built antweights im guessing your have a tx and rx.
The sabertooth 10A controllers are pretty good, especially for low power use on some cheap drills. You can get those from technobots. We have our own custom Robo Challenge Sabertooth speed controller's, but for your use the standard versions will be just as good.

you can use the drills batteries to get started too - although 12v stick packs are readily available cheaply on ebay.