One of my favorite places to find robot parts and platforms are garage sales yard sales and flea markets. Particularly because you can find old outdated (At least by today's standards) electronics that will work perfectly well with other control electronics. For example I was at a garage sale and the person that was running it asked me if I liked to tinker so naturally I responded yes and he gave me a box of RC cars that must have been at least 20 years old! For FREE! So I took the box home tested the two cars that were in the box, then I found the reason he gave them to me. They the RC system didn't work. So I hooked up an Arduino and a motor shield and after five minutes of coding I let it loose and it zoomed around the floor! It works now! Now I always look at garage sales for these types of things that people never use but work fine with a little work.