I'd look in your local area, first, and look for a smith who is an S&W authorized repair station.
Call S&W, they might be able to tell you who in your local area is....
The 66 is a discontinued model, S&W may no longer work on them, I don't know. There are several pistolsmiths that advertise nationally, and certainly one of them could fix your gun, but their cost and wait times are high.
Try local first, even if its a couple hour drive, it may be the best way to go.
(note, in today's virus restricted times, having the gun shipped to a smith might be your only practical option.)
Call around, see who's still working, and what they'll accept.
Good Luck!