The name that springs to mind for letter and number punches is Pryors, IF they are still trading.
Have you looked at Cromwell Tools catalogue? Probably relatively expensive, but they are supplying industry for heavy duty use.
Ditto for Buck and Hickman.
Does it have to be stamped? If not, you can buy a hand held engraving tool fairly cheaply, (Aldi or Lidl offer them from time to time) or again try the firms mentioned above, as well as some of our M.E.suppliers.
Howard