Hi pvphoto,
Like they said, put a want ad in the list's classifieds section.
FWIW, it don't have to be a 1980 tank, or even a Special tank. Any XS650 type, any year XS650 tank will drop straight on.
Caveat. Early tanks ( I think 1973 & earlier) have a narrower shape that I'm not sure drops onto later bikes and the resto fanatics have driven their price through the roof.
Note that the Standard and Special filler caps and gas taps are different enough that they don't swap.
OTOH, the Standard tank does hold a gallon more gas.