All Good advice above.
First, Check with Larry Poppert in Pennsylvania; He's a great guy to do business with, and He may have brand new, old stock, forged hammers/triggers.
Poppert's Gun Parts Main Menu
If not available:
Here's a couple good used K frame FORGED hammers: the first is a .375" combat/semi-target, the 2nd is a .265" standard FORGED hammer.
Smith & Wesson S&W "K" frame .375 FORGED Hammer : Other Pistol Accessories & Parts at GunBroker.com
Smith&Wesson S&W "K" frame .265 CC FORGED Hammer : Other Pistol Accessories & Parts at GunBroker.com
Magnum Mark (above) is quite "fussy" about methods of payment, however, I've bought and received parts from him, and the parts did arrive.....eventually.
If Your model 10-5 has the .265" hammer already, personally, I'd move up to the .375" forged hammer. Easier to grasp with the thumb, fwiw.
I'm not sure if S&W will send out a new FORGED hammer....they seem to stock only the later MIM hammers/triggers in the regular parts section.
If the gun was returned to S&W for a new hammer, then You'd possibly receive a forged hammer....not sure.
Good Luck!