Kits for model engines or tooling?
It's getting tougher. There used to be Power Model Supply (blew himself up) and Coles (faded away after the partiachs death, afaik) that had great selections of locomotives, traction engines, stationary engines etc. All gone now.
The chap in Elmira took over from Ted McJannet (of Golden Triangle Model Engineers) and he had some nice casting kits. I built his 1/4 scale Perkins hit and miss and the Rider Ericsson hot air engine. I thought there were good well done projects and would recommend them if Canada Model Engineering still sells the kits that I think Ted was involved in developing.
Castings sets frequently come up for sale in the classifieds, where I've bought all mine. I've got so many under benches its frightening.....better get busy or the market will be flooded one day when the kids go to clear it all out. Stuart Major beam, Seal 4 cyl engine, vertical hobbing machine, two locomotives and three tractions engines.
The UK is centre ice for model engineering with lots of suppliers, Reeves, Stuart Turner and so. Check out how many businesses service the sector:
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Many kinds of engines you need castings for an authentic looking model (radials being one exception that comes to mind) but if its tooling you are after, I'd just go bar stock (even it means getting a hunk of durabar)