ebay. make sure u check the feedback of the seller and look and see if he sold the product before and what the customer says. if u want to get an OEM part its going to be EXPENSIVE. my bro changed the color of his iPhone for about 60$ from a kit he bought on ebay. but the only thing is be warned if u didn't do it before, make sure u go on youtube and follow the video, and make a sheet of paper and label the screws and put circles in to match the label or print out an iPhone parts sheet if there even out there. some places sell them for 1$. theres about 30 screws (rough guess) that are very tiny. and about 8 cables you have to undo in the process. the hardest part is popping the glass out of the front. ur better off buying a kit with a display in it already thats what my bro did, not sure if its a retna but it looks pretty good. he broke his glass popping his original one out. good luck. also check la computer company out they do it, u may be able to mail it in. there damn good and reputable if u don't feel like dealing on ebay.
good luck, this is why i bought applecare lol. so i didn't have to rip it apart, I've done it before mind u, but these phones are sooo easy to break if u don't know what ur doing inside of them, and are not careful. u can put down a connector into a connection wrong and bam its done and ur out however much it costs to replace it. and most of the places on ebay are over seas. and its cheaper to buy from hong kong or china. u just wait longer. and returning it is tougher not only distance wise but also language wise.
Thanks guys. I was asking I just got my new iPhone 4s and there are spots on the back of it that look like paint is missing under the glass( tiny little dots) and i don't want to take it back to apple for them to replace it. so I wanted to buy a new back cover but wanted as oem as possible