Hi Rob and welcome to the forum.
Off the top of my head, I don't know of a specific one, but if you look hard enough, I'm sure you'll find a replacement (they were prolific).
You mentioned Numrich.........also check Brownells World's Largest Supplier of Gun Parts, Gunsmith Tools & Shooting Accessories - Brownells Midway Shop Shooting Supplies | Reloading | Gunsmithing | Hunting gear — MidwayUSA
GunBroker has a lot of parts listed too Guns for Sale - Online Gun Auction - Buy Guns at GunBroker.com
I even found some sights for a Model 27 on eBay
Good luck, it sounds like a fun project to make that old M&P right again.