Since the OP hasn't told us what make and model of shotgun this is, it is hard to say if a Remington 1100 firing pin could be modified to fit.
I would advise Snipe Hunter to tell us what make and model of shotgun he has, and to provide a pic of the broken firing pin pieces if possible. I have found original firing pins from sources such as Numrich (Gun Parts Corp.) and other parts dealers. I see various firing pins quite often on Ebay. Gun shows are also often a source for firing pins that are correct, or could be modified to fit. Someone here might even have spare pins from a parts gun. However, having a firing pin custom made will likely be the most expensive option.
But as Stan says, making a firing pin for most doubles is not very difficult for any gunsmith or machinist who can operate a small lathe, and knows how to harden and draw a piece of common oil hardening O-1 drill rod. The first firing pin I ever made broke on the first shot because I drew the temper to light straw color, and the tip end was too brittle. Since then, purple or blue has worked well for me when using 0-1. Some people say that using a grade 8 bolt for material works well, as it is tough enough for most firing pins without subsequent heat treatment, but I have not tried this yet. It would be nice to have both broken pieces to be able to make it the correct length the first time. And it should be noted that the discs will probably not be interchangeable from right to left, so they should be marked upon removal.