Welcome to the Forum. gunpartscorp.com is your best bet. There are a ton of 32-20 and 38 M&P hammers out there, but you will have to match the era of your gun, since changes were made over time. Your revolver is a 32-20, 4th Change. The 32-20 4th Change was made from 1915 to 1940 with at least one change in hammer design over that time period that I can recall, so you will have to look for a period hammer from around 1920 or later. All hammers for M&Ps were case hardened and should fit.

Good luck on your hunt.

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Gary
SWCA 2515