Economical Robot Chassis Kit VerA
I purchased a bunch of these from China. They have simple DC motor drives and work fairly well. Plenty big enough for a 830 point breadboard.
There seems to be at least two basic versions of the same thing. These have a lot more mounting holes but were more likely damaged on the trip from China.
I have the site set up so that you can't order them unless I have them ready to ship. So if you want them in the next few days, order them from here. If you want to save a buck or two and wait...order them from E-bay.