You can get high quality replacement screens from several reputable vendors (not Amazon or eBay). These usually contain an original Apple LCD with a copy glass on top. The various innards, such as backlight, touchscreen, flexes etc. will vary from original to copy depending on the supplier. You also want a replacement screen that was assembled with a cold-glue process (like Apple does) as opposed to a hot-glue process (yes, THAT hot glue!).
Of course, if you want a true original screen, then the options are Apple Store or buy a used device. However, keep in mind that the used device you buy may also have an aftermarket screen as probably 25% of phones suffer screen damage at some point in their life.