I would personally caution against no-name knockoff batteries, like you find on eBay. They may be cheap, but they can fail with spectacularly bad results.
I always get stuff like batteries from Seidio Online
. Reputable, great customer service, and they have a presence on this forum.
Standard battery: 1400 mAh.
Seidio (Innocell): 1600 mAh ($50)
Seidio (Innocell): 2600 mAh ($60), comes with hump-back rear cover.