Authentic ones were never made available to the public, and they were adamant that they never would be, as they only wanted them to be worn by the team. That said, like others have said, there are good reproductions out there. Depending on which jersey you're doing up, though, there are some important specifics.
Home, 50th, and Stadium Series - all had an embroidered Snider patch all season
Away - had a screen printed patch for Sets 1 and 2 (so if you're doing it WITHOUT the 100th patch, the Snider patch should be screen printed)
Hall of Fame Game - obviously you'd need the Hall of Fame patch as well, which isn't available, but that specific game had an embroidered Snider patch as well
If you truly want an authentic patch, look into a Meigray GI that could be stripped and just use the patch from that.