I have one of these Gorilla Gadgets for my soon to be retired Droid X. It has been a good battery for my X, and I just saw that they are offering a 4500 mAH one for the SIII. Only drawback is the supplied back is offered in white only, and it does not mention NFC capable ( I will assume it is not).
Gorilla Gadgets Battery
Gorilla Gadgets Battery