Try e-gunparts.com
It was made by Mossberg. We have new firing pins. sales@countrygunsmith.net
Field strip the weapon down to the bolt and bolt carrier and then use a steel punch to drive out the firing pin retainer pin on the back edge of the bolt. The firing pin and firing pin rebound spring will drop out into your hand. To reverse that, put the bolt face down on a table and force the firing pin/spring forward into the slot to allow the retainer pin to be replaced. Reassemble weapon as usual.
I can't find a listing for the Western Field Mod 15k. The Mod 15 is a Mossberg Mod 80, the 15A is a Mossberg 83, and the 150 is a Mossberg 183k but all of these are .410 bore, not 12 ga. Mossberg did make most of the different models sold by Western Field, so by looking through the different diagrams of appropriate Mossberg shotguns you may be able to identify the correct model. With that information, GUNPARTSCORP.com or other parts warehouses may be able to supply the correct firing pin.
Find a good gunsmith or machinist
50-150 USD or so
A firing pin that is mounted on/in a hammer
Firing Pin Block
I don't know where or who made it, but I found a firing pin for a very old single shot mossberg 22 out of MO. I'll look for the address for you and you don't need a firearms permit to get it sent thru the mail to you!
What should Firing pin protrusion measure?
Like it says, the firing pin rotates as the action cycles.
By action of the hammer striking the firing pin, forcing it into the primer.