I actually bought the Seidio extended battery with the back door that accommodates it:
Amazon.com: Seidio BACY35SSGS3-BK Innocell 3500mAh Extended Life Battery for Samsung Galaxy S III - Retail Packaging - Black: Cell Phones & Accessories
I've read on this forum that you can call Seidio and order a back door "a la carte", but you have to call Seidio to do so (i.e. it's apparently not for sale by itself anywhere, including Seidio's website, last time I checked).
I went ahead and bought the battery & door combo from the link above so I'd have a backup battery in case...well, just in case. You never know what's going to happen to/with a phone battery, particularly a new, aftermarket version. But the Hyperion has been a freaking steal, and I haven't even bothered to power up the Seidio battery to see how it compares.