You might have a little problem finding rims, but the equivalent height tire to an 18" tube type is a 19.5" tubeless. A lot of the old bread trucks and other step-vans ran that tire, for some reason. That option also eliminates the problem with multi-piece rims. Tubeless tires need one-piece rims to seal the air properly without a tube.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!