I recently purchased an extended battery from Amazon.
I haven't heard of the company before but for $17.99 it was worth a shot.
The battery snapped in and the battery case fit nicely on the phone.
I really like the black mat finish and the battery lasts 70% longer.
Here is the link I purchased off amazon:
Amazon.com: RND Power Solutions High Capacity Extended Battery for Blackberry Curve 3G: Electronics