First thing is to measure them with a caliper.
Sometimes you can luck out and find them on Ebay.
Do a search on all of Ebay, not just in the U.S.
The last one I needed for my childhood JcPenney box, I ordered from China via aliexpress (you can also try alibaba).
I ordered the whole tire & wheel assembly thinking I'd transfer the tire to the original wheel but the whole assembly ended up fitting perfectly.
I think I paid around $10 shipped for two assemblies (I ordered two in case I destroyed one).