How much was it? I know it was some time ago.What's wrong with yours? I bought a tank at Michigan Truck Parts in Westland, Michigan years ago. Do you want to stay with something in the 80 gallon range? Frame rails on most stuff in the medium duty kind of size is fairly standard. Might be able to make some thing work, but smaller tanks are more common.
And what chassis do you have? Chevy, Ford, or Freightliner (although that wouldn't have a "gas" tank)?