Not quite sure what you mean by "castings for rivet closing tools" specifically, but Chronos sell Snap Head rivet setting and forming tools. Are they the same thing? I've just bought a pair for 1/16" Snap Head rivets.
Grateful thanks, Jason B. The Atelier ones look perfect – but I shall have to enquire if they ship to the UK. Otherwise, it will have to be the "Polly" one. I remember some years ago someone did a kit – not Hemingway – and the actual casting was made from a hardened steel. Might have been Bruce or Maidstone.
I bought my castings direct from Atelier. The castings are very nice quality iron. However, I have not found that the resulting tool is very satisfactory, there seems to be too much flex in the castings for copper rivets. I was warned off after I'd ordered mine and my correspondent recommended adapting a pair of compund action pliers instead.