Might try going to an agricultural focused tire shop, one that also does tractor tires, because sometimes farmers have those classic old farm trucks that they want to keep authentic vintage and they'll likely run the same size tires you're looking for. Otherwise, if you're not trying to stay authentic with your bus then I'm with Jolly_Roger_bus_223 that a full axle swap has a lot of advantages including better brakes and also you may likely benefit from better gear ratio for fuel economy.
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