Okay, I've got a sort of lead for you, but it'll probably require some leg work. I ran across
this list of battlebots motor specs. two motors in there look like a good idea for you. The Dewalt 24V hammerdrill motor with gearbox, and the NPC 64038, which you'd have to gear up to twice the speed.
It says the dewalt costs $85, but I have no idea what the part number for all the stuff is.
EDIT: So I was bored and did the legwork for you. The drill model is DW006K-2. Get an account on
www.dewaltservicenet.com and you can gaze confusedly upon the exploded view of the thing. It looks to me like the parts you'll need will run you from $100-$150 per motor. Depending on how much case you want around it.