LM7805 in TO220 package (Radio Shack) from a fuse protected 12V source, you've got plenty of heat sink available I'm sure (120 pounds of it), but you shouldn't need much. Careful, the heat sink tab is connected to ground which means you better isolate it from the robot (FIRST rules I believe tell you this is a must). Don't forget an output capacitor, the LM7805 is funny that way sometimes, but other than that, it's solid as a rock. I love talking about my favorite chip, hope you get to use it!
http://www.ugrad.physics.mcgill.ca/Specs/LM7805.pdf