Have you called S&W? I've one of theirs, the chrome lined barrel is very helpful since I got it to shoot the cheap 5.45x39 corrosive surplus ammo (back when the price per round was about the same as 9mm FMJ), now that the cheap ammo supply has pretty much dried up, I don't see much point to buying 5.45x39 ammo that costs more than 5.56 or 7.62x39.
If S&W stopped making them and doesn't have any in inventory you will be stuck with the used market, or one of the "second tier" makers. I've been thrilled with uppers I've gotten from Model-1 Sales (including a 5.45x39 11" upper for my SBR).