If you can't find a good used one, you can have yours completely rebuilt like new at Moyars Fuel Tank Renu in Greensburg PA. 1-800-328-9550. They specialize for antique car and truck fuel tanks. They say that no matter how old or how bad of shape it is in it can be rebuilt to like new. They ain't cheap, but it might be worth a look at.
www.gas-tank.com. I ran across them several years ago when I thought that they were going to be my only choice for my 1974 wagon when the tank rusted through, but luckily I was able to treat my with POR-15 and it is still (so far) holding gas, but if and when my tank goes south again, I'll ship it off to them.