Hi Edward, I have never seen new gas tanks for the Type 3 offered. The main options are to have your original tank cleaned and then use a coating to seal up the tank. You can get a gas tank cleaned at most radiator repair shops. The most common sealant that I am aware of is the Por 15 gas tank sealant, which is widely available.

Option #2 is to find a good used tank. Being that these are all going to be about the same age, you might be back to option #1 after buy a used tank!

Hope this helps, Andy