search online for truck wheel dealers if you want new. The 19.5 sizes are used extensively by the lighter medium duty trucks and panel vans in the 10,000 - 18,000 pound weight class. The size was also used on some sizes of school bus chasis. New is not going to be cheap, maybe $200-300 for a painted steel wheel.
For used, there are salvage yards specializing in medium duty trucks and/or school buses. I see several along I-44 on the run from Joplin to St Louis, don't remember names. There have to be used truck dealers and salvage dealers all over the coountry.
