Rio Grande's Impressions department can order those for you. You can
also call over and request to speak with someone in the impressions
dept. You will need to send in artwork. Keep in mind that when your
design is shrunk down and is stamped into metal, you need to be able
to read it. We often see designs that are too complicated to stamp
well in metal.
Here is a link for custom marking stamps.
Technical Support Specialist
Rio Grande, A Berkshire Hathaway Company