Sorry, I got no ideas about replacement pins. I just tear apart, errrrrr, "gently disassemble" my bands and then cram them back together.
If it's a rivet band I bend the midlink until it clears the bar holding it in place, and take it apart. Then I reattach the links to recreate the band, then hold it in my palm with "inner" side out, and use a small screwdriver as a punch to bend the midlink back to where it wants to be.
Crude, yes. But it also often introduces a very mild deformation to the midlink that looks exactly like the band has been well used and bashed against things for many years. It looks completely authentic.