A good place to start with a scrap robot is with old toys. Kids discard R/C cars with broken electronics (lost transmitters) that you can use as wheeled chassis. You probably don't want to keep any of the electronics, but you may be able to get a wheel, chassis, and gearmotor combination that works that you can build off of.
Another great thing to hack is the Robots from Wowwee. They are designed to be hackable. You can find a bunch of used ones on ebay, but probably want to find a discarded one, not one for sale. If you are going to hack it, broken is fine, as long as the mechanicals are intact.