My tank, 32 gallons, cost me $127. They do have a fairly comprehensive range of tanks at Plasti-mart.
One of the places who made/make a lot of the tanks for the RV's is
Inca Plastics Inc.
It would not hurt to contact them to see if they were the original supplier and if so if they can make one.
I also wonder if there's a way to go on the Airstream website, get a list of their dealers, and do some emailing to see if anyone has one in stock sitting on a shelf they'd want to unload. Just a thought. There's not THAT many dealers out there.
Let us know what you decide. In the meantime you could have it welded back up - that is not that big a deal to do and repairs are generally stronger than the tank was at that spot. Then you can take your time deciding what you are going to do.