In your tire search, don't forget the similar-vintage 5-tons as a source. The current feeling is to go with 6 Supers.
Those that do will usually have 10 or 11 mounted 11.00-20's.
One could make a good deal, swap the 10-bolts for 6's, and be happy.
It all depends on where you're looking, there are lots of M39-series trucks in Europe, especially in France from what I see.
Don't forget, Deuce wheels will accept up to a 12.00 tire, those might be more available. They were used on the M246 5-ton wrecker.
DG