I have always heard that S&W has a lifetime repair policy. A buddy of mine sent a used one in w/aftermarket sights(semi-auto) complaining about accuracy. They sent it back to him w/new adjustable factory sight and barrel FOC. Don't know if that's true now a days???
S&W will repair one of their pistols if it is under warranty for free, but if you're not the original owner, no. The cost to send in a pistol for just that one repair would be expensive because of the shipping costs there and back. Brownells has S&W parts in stock. I just looked up the cost for replacing the rear sight blade on a Model 14 and it only ran around $12. You can also check with Gil Hebard Guns and see if he has any S&W parts still in stock.
I should add that they did fix a safety issue for me for free. I had sent in a Model 25 to be reblued and mentioned that the cylinder catch seemed worn out. They replaced it and the associated parts for free, but that was over ten years ago.