There are fuel and water tank manufacturers all over the place - if you know what you want, ask around and I’m sure you’ll be referred to a local shop who fabricate one-off tanks of all types and sizes all day long. Only when somebody has something off-the-shelf does paying for shipping empty space make sense!
If it’s rusty, and it’s not leaking, it is FAR better to replace it now while you’re in control of the system than later, when you won’t be. I’ve done this twice: once when I bought the boat, and the surveyor found the leak, and the seller had to jump through hoops to get a new tank in place quickly. The second time was 20 years later when I noticed rust and what may - or may not - have been the very beginnings of a leak while the boat was on the hard.
I replaced my fuel tank for the second and final time in my 34-year history owning the boat in, I think, 2011 with a stainless tank that was fabricated locally based on a simple drawing I made of the installed mild steel tank. It was ridiculously expensive, yes, but there was no shipping cost and I will never wonder about the tank’s integrity again.
I bought a polyethylene Moeller tank for my head discharge several years ago because one of their standard sizes fit miraculously, perfectly under my V-berth. The shipping was a bit steep considering the price of the tank but they were able to put the intake and discharge holes exactly where I wanted them, and that was all the customization necessary. The entire transaction took place in my living room after I had the measurements of my V-berth.
