normal sized 3S (like the many 2S hardcase lipo size) i havent seen many to choose from at the moment. reedy, LRP or trinity have them that im aware of.
If you can use soft/caseless lipo then you have cheaper option of using the turnigy nanotech (traxxas replacement battery) I got the 4000mah 40c ones, and the fit well but I put some cloth tape on the top and bottom to protect the shrink wrap from tearing.