7.50x20s are sold by Universal Tire and other vintage tire sellers. They are about $250-300 per tire. They are made by Specialty Tire of America (in PA, IIRC) and are not hot sellers so availability can be spotty.
Modern tractor trailers also use 7.50x20s, but not sure they would fit the rim of a civvy passenger truck.
For the love of God please do not put NDTs or NDCCs on anything you will have on hard surface roads.
