For robot parts check out http://www.robotzone.com and for cheap geared motor drive units check out http://www.robotcombat.com . I bought two wheelchair drive motors on ebay yesterday for $40. They're 110rpm 13amp 24v units. A dual channel speed control from http://www.vantec.com can drive them. They have serial PC interfaces that you can use to drive servos (or speed controls) from a PC. Good luck writing the application, you'll make millions if you can win DARPA!
-Tom