You can use most 5 cell NiMh packs out there, most range from 1100-1400mAh. Depending on your battery box or what mods you've done you can either go with flat packs or humps.
I've been using this HPI one link it has served me well an improvement from the stock one but as jetmechG550 said You can use most 5 cell NiMh packs out there try get something close to 1400mah
I saw those, tell me...what kind of run time do you get off them. Are you running a single steering servo? I'm interested in the quality of these packs, the price is nice.