Look on hobbyking.com. They have tons of lipos to choose from. Figure out the dimensions if your battery area and just look for a 2s lipo that is about the same size or smaller. I think that the 2200mah 30c batteries should just fit. You may ant to go with a slightly smaller battery just to be sure though. The turnigy and zippy lipos have a pretty good reputation and a lipo for 1/18th scale can be had for about $10 a piece.
Hope this helps
Shea