Fuel tank replacement

I think my tank has a leak. I filled it up yesterday and the fuel vent is clogged and spilled gas back into the bilge. Well, on the way out I hit the bilge pump switch and she pumped out brown water. I smelled the discharge and it smelled of gasoline. I am thinking its in a weld toward the top of the tank because after a while the smell disapates. There is an odor of fuel from the reat lazerttes. Anyone know where I can get a cross poly tank for the 1987 V20? Dirk