you can’t get new OEM from anyone but apple.
you can get used OEM on ebay, either stand-alone or as part of a whole phone.
there are lots of places to buy 3rd party screens - ebay, ifixit, injuredgagdets, amazon, etc, and they will vary in price and quality.
i get mine on ebay (vcellparts is my preferred seller), but like all of non-OEM stuff, YMMV depending on who you buy from.
i know it’s on the cheaper side, but i tend to get new screens (no cam, no mic, no button - because i can usually re-use those parts) for under $25 for 6/6s/7.
ifixit has some great repair tutorials/guides - definitely pay attention to them if you’re not well versed in repairs.