Google Infuse Extended Battery immediately shows 5 sets $13 to $30. Each includes a battery and a replacement cover (the battery is too big to fit without replacing the cover).
One problem is you'll have a hard time finding a case to fit.
Lots of info on this forum - search "extended" in title or 3500 in title.
(I'm not trying to knock, anyone down about searching, just pointing in the direction of the info I know about)