I researched on the web for the iPhone Back Cases & found that the back cases are sold for ~$50-$60 range. But found that all of them were copy and not genuine. I checked several repair shops and found that everyone is offering back case replacement, they did not mention if it was a copy or genuine on the website. When I called up the repair shops, I was told that the Back Cases are not Genuine and is a High Copy.
I found Cupertino iPhone Repair, they said clearly on the website that they use all New Genuine Apple Parts. They offered all Repairs but they did not offer the back case replacement. I called them up & was told that the Genuine back cases are not sold as it makes it very easy to create the counterfeits. Also they pointed me to the page on their website which explains why back cases are not available. I asked them what about other parts being offered by other repair shops. They said everyone in the market is using Copy Parts as Apple Parts are twice as expensive than the copy.
Original
I again called up other repair shops ubreak, Mission Repair and found that indeed that they were using the Copy, they call it OEM and not Genuine Touch Screens. I checked their website and did not find the word Apple Genuine mentioned anywhere.
So if you want to do back case replacement, you do not have choice except using the copy.