I see some kits on Ebay occasionally but I have no idea what is a good price.
https://www.ebay.ca/itm/LIVE-STEAM-...811180?hash=item3d5d0802ec:g:S-cAAOSwGhZZsxE1
I was poking around the PM site. I see some of their kits are on ebay too but, depending on the model, it might be like 150$ for the kit + 60$ shipping. Maybe shipping to USA address would help if you have those connections, but this is kind of in the nuisance range not really worth a drive unless you want to or can collaborate with a buddy on larger order.
If you wanted to design your own engine that's another matter but my guess is by the time you made the pattern & located a small scale CI foundry that would actually do it, you would probably have sunk as much or more than a kit. It may seem expensive but I'd bet they kit distributers are making squat, more doing it for the love of hobby.
Another tact is to write a reverse (wanted) ad in a model engineering forum. There might be someone who is willing to thin the herd as many of these folks are getting on in age. But people know what the good castings are worth & you still have shipping which wouldn't be cheap for a couple pounds of CI coming from England say.