Anything X 16.5 can be hard to find these days. Ag suppliers are your best hope. If you want to change wheels (not stupid, but not cheap either) you need to call someplace like Southwest Wheelhttp://http://www.southwestwheel.com/
and talk to them. They may have a direct listing and be able to direct you to a local dealer. Many of the larger tire dealers can do the same, but you have to find THE GUY that knows how to work the catalog.
You can also hunt on line like Craig's list and maybe get lucky.
Do you know of a local wrecker that specializes in trucks??
Good Luck, it took me years to collect the right wheels.