Wow, what a vintage old bus. Finding the old 7.5 x 20 tires may take some effort but they should still be available. Forget automotive tire shops, skip Sams and Costco. You *NEED* a commercial tire shop, these are likely tube type tires mounted on split rims and require some specialized knowledge to replace safely.
You'll probably have an easier time finding 8.25x20 tires (the next size up), and you'll also find 9x20 and 10x20.
It's hard for me to see in the pic, but if you are running the Dayton type wheels (the rim is just a ring, the spokes remain part of the hub), you can swap it to 22.5" wheels easily. They should be a direct bolt on swap. 22.5" tires are very common and don't use tubes. Any commercial tire shop will handle these easily.