Economical Robot Chassis Kit VerB
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 fewer mounting holes and smoother edges so they were less likely to break 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.