I have a 250 GMC which also has the 18" wheels. I recently found a set of NOS Nylon Bias 700x18 by doing a search on this site.

http://www.adhuntr.com/

All kinds of weird stuff can be found all over the US on that site. And since I have been in the trucking business for over 51 years, I can usually figure out on how to get stuff moved. And nylon tires don't blow out with age like radials. As long as they are either not mounted in a warehouse or mounted and kept inflated, they are still fine.