Welcome!

The only gas tanks I have seen were for '83 to '85 D-50's.
I haven't found any for the later models, though maybe someone else here has.
You could try car-part.com, where I'm sure you could find a good used tank at a reasonable price.
If that doesn't work, you could always try e-Bay.
Good luck to you on your search.

Nice truck, by the way.