I don't know of availability, but about 40 years ago I had a leaky gas tank.  I removed the tank, sanded it to bare metal, then coated the underside and up over the seam with fiberglass cloth and resin.  I knew some gas stations had fiberglass tanks so I figured it was worth a try.  It worked fine.  No leaks and I drove it like that for several more years until I junked the car.  It never leaked again.