If the Hammer Mounted Firing Pin is still doing its job, making good dents in primers and has no apparent chips or dents I don't see any reason to replace it. You mentioned that it has some side to side movement and some is normal and required, as long as it is not excess.
Hammer noses are usually available at local gun shows and eBay. They are were also being sold by Ron Powers last I looked. Parts resellers like Numrich Arms, Bob's Gun Shop, Jack First Gun Shop, etc. usually have them - sometimes used. You could also try S&W but don't hold your breath.
They aren't expensive and usually sell for somewheres around $20 or so. If you do buy one, get yourself a new Rivet also. They do break from time to time but it's been my experience they usually last a very long time and most Revolvers probably still have the original ones. So while not an uncommon repair, it's something that you don't normally have to replace on any routine basis.
