The firing pins in the 21 are one piece with the hammer.
The hammer(s) itself are usually replaced if the firing pin portion of the part is damaged for some reason.
21's are not a particularly difficult gun to work on,,the parts not needing much fitting,if any most times.
But it can be a difficult one to dissassemble and re-assemble if you're not familiar with them. Not a good one for an unexperienced person to try out on.
Briley in Houston comes to mind.
http://www.briley.com/index.aspxI'm sure others will give references.
What indications are there that the gun would need 'new firing pins'.Just wondering..
