Like others have said if you really want an Authentic Jersey you have to go through nflshop, bills store, or pretty much any sporting goods store that sells nfl licensed merchandise.
If you don’t want to spend a ton of money(authentic authentics are not cheap) you can go the knock off route like DHgate or the many other China sites.
Some say you can’t tell the difference but those people most likely have never saw a real authentic jersey. One dead giveaway is the crease that goes up each sleeve to the collar and the wrong teams numbers used. There are a lot of Bills fans thinking they have a real Bills Jersey with patriot* numbers.