The original 18" tube type wheels will be available, but scarce. Another option would be to look for some 19.5" tubeless tires and wheels. A 19.5 wheel and tire will have the same height as an 18" tube type. They were used frequently on step vans like bread trucks and other similar vehicles. Check with bakeries in your area to see if they have old wheels and tires on hand, or find out where their retired vehicles might have been salvaged. 19.5 tires are in use now on motorhomes and other medium-duty trucks and they should be more available and affordable than 18's.
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!