I'm going to give you the response you don't want to hear but the best option for tire availability would be to go to a 16" wheel. Assuming you have the 8x6.5" bolt pattern (which I don't see why you wouldn't) you can use bud wheels from and Chevy, Dodge or pre-99 Ford dually. That's what I did on my 63 1 ton dually to get a great selection of tubeless radials. I personally went with 235/85R16s because it is one of the most common dually sizes.
I did a quick online search and it would appear there is a very good tire selection available for 18" wheels but tires for 16s were considerably cheaper.
Try
www.tirerack.com and see if you can find what you're looking for but don't be surprised if you can get a better deal at a local tire shop. (shipping charges on six tires add up real fast)
Just my $0.02 - SwampYankee
