McMaster Carr or Grainger sell them in all sizes. The only bad thing is that they cost more than a new compressor. The tanks by themselves are way out of line in my thinking. I paid $160 pluss shipping for a 20 gallon tank but I had a comercial V head with a 2hp American motor to put back on it.
I would think the tanks would be $350 and up. They were 10 years ago when I replaced mine. I have a 60 gallon compressor in the shop. I needed the other one for the job site. In 50 years of junking I have never seen anything larger than a used 30 gallon tank for sale.
I am sure one is out there. The shiping alone will be $150 or more.
Ive found any where from 500.00 to 1000.00. there is a place 50 miles away that fab them but i have to go thru a dealer. I have a industial motor and compressor. otherwise i would buy a complete set up. I would buy a used upright as i dont think they are as apt to rust thru like the lay down ones but i dont know that for a fact.
i have an 80 gallon upright tank in great shape,it is from my old puma air compressor the motor fried.so i bought a new ingersol uprighti'll take three for it but i hate to see what the shipping would be.pm me for more info or questions.