OEM means Original Equipment Manufacturer.. So you'd be referring to parts made by Apple Suppliers.. the actual parts that end up on the phones Apple sells to the public.. if that's the case there's only one way to get them and that's by removing them from other devices. Apple will not sell these parts to users. They could replace a very limited set of parts at their stores.. however, nowadays Apple mostly replaces devices for pretty much everything.
Unlike most other brands Apple doesn't offer parts for sale. They do have parts that they use at their stores and that they provide for repairs to their authorized repair centers. However, there's no such thing as an outlet for Original Apple Parts.
Most eBay sellers sell replacement parts made by factories who make a good imitation. Depending on the price you could get even a moderate quality part.. never like an Apple Original Part. We don't like installing non-Apple Parts.. In our experience here they never meet my quality expectations, specially the displays.
Hope this helps :)