It fit's perfectly the phone, it's resistent.
It has a 2200 mA batttery and a button to turn it on or off.
It also has 4 blue leds to show how much you have left of the battery.
Ebay search link OR Use these keywords in the search form of ebay Backup Battery Cover Galaxy S2 and set location to worldwide
OR go here -> acetoys.eu <- [thanks to daspondicious1 for the info] - ships from UK
co0kie who also ordered this case has made this great review