Hey Bill, There is an outfit called Garrett Machine & Welding that sells the adapters like you mentioned. Don't know the price, but they can be reached at 541-259-2514. There are other suppliers I've heard of over the years, but I've never bought any. I was told that someone sells them in both steel and bronze. There are also suppliers that advertise them for old Harleys, but you don't want those. They used an 18mm plug with a gasket, not a tapered seat plug like the Y-blocks and FE's.
I've been planning to try a set of the adapters to broaden the choices of spark plugs (and heat ranges) for our cool Y-blocks.
Nick Brann - retired spark plug salesman in K.C., MO