iPhone 4 Back Cover-Broken

Hello all.

My iPhone 4's back cover is broken so i want to replace it. I bought my iPhone almost 2 years ago, outside US. It seems it is pretty easy for one to change it at home but since I can't find an "original" back cover, I can't. The ones sold in eBay, etc. are all replica.

Is it possible for me to get a paid repair from an Apple Shop in US? If so how much does it costs? I just want the part that will be replaced to be %100 geniune Apple product.

Apple part number for the back cover is "922-9831" by the way...