Russ in OKC area thier is Tractor Supply. They have a large variety of tires from little to air up to solid tires . I have seen 3" to 12" from axle turn to roller bearing tires thier. This would be a great spot to start.
Russ thier is another place down off of Agnew called Caster of Oklahoma, this is were I have bought some to replace the wheels on my grinder.