The only official apple parts are from an apple store or one of the official repair centers. All parts are carefully stock controlled and not allowed to be sold.
You can however find genuine parts from stripped phones with other issues or actual real parts that have slipped off a truck somewhere
90%+ of parts found on the internet are not made by the official OEM producers and vary greatly in quality, also you usually never find them advertised as "official genuine apple" products as they will get in trouble for it.
We fix phones where i work and we have a supplier who has a middleman who gets us very good OEM parts, hard to tell sometimes if they are "real" or not but always very good quality. We have found that nearly all ebay sellers have crud parts, there are the odd good ones, but in general they are shoddy copies.
As a general rule i say if your phone is still new, and a premium model then best get it done at an apple store.... if its not so valuable go for a repair anywhere but make sure its a reputable place who will offer support if your screen starts to peel up at the sides. ( or do it yourself if you have good eyesight and are good with tiny connectors! )
In my town we have had a lot of come and go repair shops, who offer silly low prices and of course use terrible cheap parts and break phones! they tend not to last long... maybe a year before the reputation is so bad they cannot get new customers! undercutting by 50% on repair prices etc... cheap nasty £5 screens off ebay!
