hobbycity.com
they have 5000mah 20c-30c lipos for only $20 each. they are labeled as 15c-20c, but they are just mislabeled.
http://hobbycity.com/hobbycity/store..._15C_Lipo_Pack
also, for less amp draw and more power, try the 3s lipos.
http://hobbycity.com/hobbycity/store..._15C_Lipo_Pack
the 2s lipos will be ok for your setup if you gear to 40mph or less and the 3s will be even better if you stick to gearing for 50mph or less.
They say a good mechanic only needs 2 tools - WD40 & Duct tape. If it moves, and its not supposed to, duct tape. If it doesn't move, and its supposed to, WD40.