I am sure you have searched this on your own and have seen some of these already.
Cookie stamps (already made and usually purchased from private sources) are highly collectable, expensive and most likely useless for your needs.
Trying to patch random letters together and stamping is probably more work than you can charge for (altho with a steady hand you could do one letter at a time get an ok look if you don't die of boredom first lol).
Chase one or two of these down to the manufacturer, ask about food use and get bids for whichever meets your needs and build the cost into your price.
You could try the scrapbook aisle of a hobby store but those stamps are not food safe (even if you could luck across one that fits your needs).
Good luck in your search (really)....I love the look of stamped shortbread cookies with a brownish glaze brushed on before baking.