Old enough to have money
Try egunparts
In older guns....and most guns that people think of when they think of a handgun or rifle, there is a "hammer" and a "firing pin". When the trigger is pulled, the hammer is released and it strikes the firing pin which in turn strikes the primer on the cartridge, firing the round in the chamber. With a striker fired gun, there is no hammer. The striker, which is basically a spring loaded firing pin, is released, doing the job of the firing pin and hitting the primer on the round in the chamber. These guns have become very common, and you will find examples of a striker fired system in all Glocks, Springfield XD's, S&W M&P's, Taurus 24/7, etc.
Find a good gunsmith or machinist
www.leesgunparts.com I have new firing pins for that model. sales@countrygunsmith.net
A firing pin that is mounted on/in a hammer
Firing Pin Block
try numrich.com
To replace the firing pin in an H&R Topper shotgun, first ensure the firearm is unloaded and safe to handle. Remove the barrel by unscrewing it from the receiver, then locate the firing pin inside the bolt assembly. Use a punch or small tool to carefully drive out the old firing pin, and then insert the new firing pin in its place, ensuring it is seated correctly. Reassemble the shotgun by reattaching the barrel and performing a function check.
What should Firing pin protrusion measure?
Like it says, the firing pin rotates as the action cycles.
http://www.remingtonsociety.com/rsa/service
Numrich Arms probably has them, but this is an item any gunsmith should be able to manufacture.