There are a number of companies that sell the firing pins, but the surest way of getting the RIGHT pin is to contact S&W and buy direct from the factory.
They'll need the model and revision number which is stamped on the frame just under the barrel, and where the cylinder yoke seats.
If you don't want to use S&W for some reason the following sources sell them:
Brownell's.
Jack First Gun Parts
To remove the old pin, you cut and punch the old flair on the firing pin rivet until it can be pushed out. Be careful not to damage the hammer while doing this.
Before removing the old rivet, use it to select a large punch that closely matches the rivet contour, then round the tip off with a stone.
To install the new pin, lay the hammer on a aluminum or brass bench block and use the rounded-off large center punch to lightly rivet the end of the firing pin rivet.
Turn it over and rivet the other side, then back to finish the first side.
You want the same amount of rivet on both sides, AND the firing pin MUST be free to pivot up and down freely.
