ATT sells them, I posted a thread last April about how easy it is to buy one from ATT. $35. w/ free shipping.
ps ignore the photo of the battery on ATT, it's the wrong picture but the right battery.
Official MIcrosoft certified service center (MTTC) is selling them here in Lithuania. Genuine battery costs about 20 EUR here. So for genuine item you should ask at your service center first