Jerry, you may encounter several different thoughts. One goes like, buy new because anything else is an unknown. And an unknown may be unsafe. In that case you local A/S or RV dealer would be a source. Or places like this http://www.wheelvintiques.com/
or here http://www.southwestwheel.com/
The other line of thought goes to used RV wheels, which can be found at RV yards. The thought is they maybe used, just not used up. So find an RV yard like this place http://www.azrvinc.com/
I would not recommend and automotive or truck rim. Some of those are not load rated high enough to carry the load on your Ambassador.
BTW the International tag is a trim or option package not a model name.