You want brass punches - to minimize marks on the gun as you punch stuff out.
Harbor Freight used to sell brass punches, but they no longer do around here in TX. A set is typically expensive. I have one I bought from Midway USA - a 3/32" - I occasionally use it, and I have found that this particular size tends to fit most gun related things that need a punch. Sometimes, I can get by with an allen wrench instead of a punch.
Wal-mart sells punch sets for cheap - but once again, they are not brass (but are steel)