I would look at Flightpower packs (either eBay or almost all model shops that sell Heli's/Planes) as they are fantastic quality and you can get them in practically any Voltage, Capacity or Size. Otherwise 2 Trakpowers in series or any of the car race packs really (Orion, LRP/Nosram, Core RC etc).
The Overlander Tornado packs are also worth a look; good quality cells, reliably rated and in various sizes. They tend to be slightly cheaper than Flightpowers though, but are still very popular in Helis, so many places sell these too.
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