Chances are good you can buy a replacement fresh water tank from Inca Plastics in California. Visit their website, down load their catalog, and first study the Airstream pages. It might be in there.
Or you could ask an Airstream dealer to source it for you. There would be a significant mark up of course. But you would get the right tank.
David
http://www.incaplastics.com/
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WBCCI #8607 VAC Region 11
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