In my Axe 400 I use a MegaPower 11.1v 20C 2100 mAh battery. You can find it at:
http://www.helidirect.com/mega-power...lue-p-2145.hdx
I've been using MegaPower batteries for quite some time now, and they work great.
I'm thinking of upgrading to a Rhino 25C 2250 mAh battery. You can find it at:
http://www.hobbyking.com/hobbycity/s...0;
I've never used that brand though, so I can't vouch for it.
If you want to stick to Great Planes, then just find a 20C or 25C battery like this one:
http://www3.towerhobbies.com/cgi-bin...?&I=LXTVE3&P=V
Although, that one is a bit too expensive for the type of battery, I think anyways.
Shop around; see what you like.
P.S.: I would definitely go with a higher mAh. Just pay attention to the weight of the battery.