Just find an old local tire shop. Just for fun i went to a Franchise place (Colony) and asked about my old trailer tires. They had no idea what i was talking about.
Then i went our legendary local store, he not only knew what i was talking about, he knew the right size off the top of his head. I beleive I ended up with ST 225/75 15 - this aint rocket science.
I has not been that long since the sized tires this way as I have a pretty good 7.5 X 15 on my newst acquisition, a 74 Boles Aero.
You will have no trouble finding the right tires. PM me if you get really stuck.