I personally like Zippy batteries from HobbyCity.com. They are good packs for the price. Look for one that has a minimum of 60A discharge (more is better, of course). If you have the stock ESC, I'd get 2 3s 5000 mAH lipos and run them in parallel (make a jumper for one of the connectors on the ESC and parallel the batteries to the other connector). This should give you 10+ minutes of run time, but you'll have to make your own battery mounts because they're much larger than 6-7 cell NiMH packs.