Since S&W does not have the part, you can try some of the larger parts guys like Numrich (Gun Parts), Midway, Brownell's, and if you strike out there, here's what I do................
Get a hold of an older list of the Authorized S&W Repair Centers and start calling them. If they tell you they do not have them either, ask them for a recommendation as to where you might try. Keep a list of people that you call, so you don't repeat your efforts, and eventually you will find one. There are still plenty of Gunsmiths that still work on these revolvers, and I'm sure there are ones that have a few squirreled away. I have done this in the past for parts that have long since been discontinued, and I have always found them. Not saying it's gonna be a cinch, but you will eventually find one.
Chief38