"The usual sources for parts for older guns are Numrich (gunpartscorp.com), Jack First and Poppert's. One of them should have one, but it will likely require a gunsmith to fit it. "

I'd check to see if there are differences in the later old long-action hammers that might prevent their use in a K-frame from this early period. I think all of the old long action (pre-1948) hammers were the same, but I wouldn't bet on it. Does anyone know?