Tonight while doing some research on this
Moon Buggy, I found a source of possible interest to FIRST teams or hobbyists.
They have a whole bunch of stuff including wheels, controls, parts, and gearboxes from ride on toys such as Power Wheels.
http://www.kidswheels.com/
Wheels
Gearboxes
I have not gone through much of the site yet, but it appears they have a lot of stuff.
Let everyone know if you find anything of interest!