There are quite a few out there. Common Sense has a really good one, a 2800mah pack with onboard balance board. It's decently inexpensive as well. I'm not a huge common sense fan but this battery works.

I am running a King Power 2500mah pack. I've had it for almost 2 years and I can 40+hours on a single charge. I haven't got it down below 10v and I'm still getting that kind of time on it.