You can spend up to $50 at the AT&T store or you can search online. I bought a bunch of them only because I found some OEMs for as cheap as a few bucks but also found out many of them are junk or used. However, I did stumble upon one online store that always sends me top quality perfect products for my BB including batteries for about $10. Do a search for "SFPlanet".
"Happiness is a function of accepting what is." - Werner Erhard (from Refdesk.com)