Measure the heights of the letters/numbers to determine the stamp size. Standard, popular sizes are currently 1/16", 1/8" or 1/4". We sell those sizes (but you'd have to buy a few of them to meet our minimum order), and you should also be able to find those sizes at a hardware store such as Harbor Freight.
We sell our
metal stamps for use primarily on copper and sterling sheet, but they are also suitable for harder metals such as steel; they'll just wear out faster if they're used on steel often.