Choosing a battery...

[MetalMan]

It kind of depends on your budget. The lightest weight/highest energy and power density will be Thunder Power. Then there are the Hyperion/Polyquest/Poly RC/Flight Power/etc. packs that are made from Enerland cells (the 20C packs) that would be another choice.

To be able to get a capacity of 6Ah with 5s Lipo (if that's the voltage you're shooting for) you'd need more space. If you go with less capacity with 5s Lipo, then it'd be more feasible. With 4s Lipo, you can get a higher capacity (obviously).
Here's a pack that would fit (Enerland cells):
http://aircraft-world.com/shopexd.asp?id=3765