Just for the heck of it, I have to ask if you have tried Mossberg? Maybe you would have to send the rifle back, but factories often hold back parts for their own repair shop rather than selling them to the parts places.
Now, IIRC, there are two types of firing pins on the Mossberg 800. Both are in two parts, but the setup is different. The early one has a large rear section which includes the sear engagement part, and a front pin that is swaged into it. That swaging, once broken out, can't really be redone. It might be possible to drill the rear section out (it is harder than heck) and insert a new firing pin and hold it with a cross pin.
The newer type has the front of the large section and the front firing pin machined in one piece, with the sear engaging part being a short stub that is pinned on the back of the firing pin. That type could be machined all the way, just copying the original, and using the original "stub". It should be a fairly easy job, using drill rod, then hardening it when you are done. The two types are interchangeable.
So, if your firing pin has a crosspin in it, it is the second type. Note that the measurements are pretty critical; the large part acts as the firing pin stop, and its rear engages the sear, plus the firing pin itself has a taper that needs to be right. That kind of fitting is not hard, but it takes some study.
(Believe me, nowhere near as complex as making a Mauser firing pin!)
Jim