I dont believe you can buy a new tank. You either have to get a good used one from some great member here that will part with one or you can buy a new mustang 20 gallon tank and relocate the filler neck.
Here is a link to another thread here with more info
https://vintage-vans.forumotion.com/t24684-fuel-tank-64-travel-wagon
Here is a link to another thread here with more info
https://vintage-vans.forumotion.com/t24684-fuel-tank-64-travel-wagon