OP, check out http://applepartsstore.com/. I replaced the black back on my 4S with a white one from them and couldn't be happier with it (black 4S with a white back now). I started a thread detailing it here https://forums.macrumors.com/threads/1279482/. I'm not sure if their parts are definitely OEM, but I couldn't tell a difference between the stock black back and the white one I got from them.