unfortunately, the 16.5 inch tire is getting harder and harder to find as the soccer mom club keep asking for the "cooler looking" large tires in the 20+ inch class.
i am looking at nothing under 18 inch tires being available for new purchase within the next 10 years.
I think you are stuck with BFG's and some of the Interco tires (Super Swampers). I was having the same problem you are having when I had my F250. If you are going to keep the truck through a couple more tire changes, it would probably make sense fiscally to just go with a wheel size change. Because of what Tom says you should probably go with an 18" wheel just to be future proof.