Just buy SGS2 original battery at Samsung Service Center. It's cheaper than if you buy it at retail store, and guaranteed you'll get original one.
I don't know in other country, but in my country, we can buy spare parts and "stock" accessories at Samsung Service Center.
Spare parts: back cover, bezel, LCD display, mainboard, etc. -> so for example you want to change ur black SGS2 into white, you can.
Stock accessories: those that originally inside the box like USB cable, headphones, battery, wall charger. Other accessories like HDTV adapter, etc is not available.
2007-05 Dopod D810
2008-01 Dopod 900
2010-12 HTC Desire HD
2011-06 Ivio Icon Pro DE88 Plus
2011-07 Samsung G.S II , BlackBerry Curve 8520
2011-10 BlackBerry Torch 9850 Sprint
2011-12 BlackBerry Torch 9860
2012-07 HTC Desire V
2012-09 BlackBerry Curve 9220 , Samsung G.Note 10.1
2012-12 Samsung G.Camera , BlackBerry Torch 9850 Verizon , Samsung G.S III 64GB
2013-03 BlackBerry Z10
2013-06 Samsung G.Grand Duos
2013-09 Samsung G.Note II Duos
2014-07 Samsung G.S4 Duos