Cheapest you can find (in decent quality).
They also a a search engine that you can input the space avalaible and they will give you the results.
NEVER. NEVER. Ok, again: NEVER. Use any LiPo under 15C. It will get damaged VERY fast in any rifle.
Also, LiPo are sensitive to a LOT of things (physical abuse, temperature, wrong charger/settings, over charge/discharge etc.) If you are the kind of guy to use a cheap wall charger to charge your regular batteries, please stay away from LiPo.
Basically, LiPo will cost you at least twice the price of NiMh or NiCd. Don't be cheap and buy the lowest priced packs/chargers.