Naturally, they're cheaper on Amazon & probably ebay too. Also the Hyperion 4200 mAh extended battery supposedly fits in the Seidio cases according to another thread IIRC on the AT&T S3 forums and its a third of the price of the Seidio 3500 mAh extended battery.
Another cool thing IMO is this Seidio desktop charging cradle that fits both of the extended battery Seidio cases (Active & Surface) with the case on.
When I reread your post I realized you didn't mention anything about a belt clip, Freudian Slip on my part because that's what I want. The below link on xda forums has a comprehensive listing.