The water tanks themselves are not Airstream proprietary and anyone can buy them from a third party supplier. However, working on an Airstream is different than most other RV's (especially when body work is needed as will be the case to drop the belly pan to access the tanks) so you don't want to entrust this work to just anyone. I would rely on either an Airstream dealer or one of the many excellent independent shops around the country that specialize in Airstream work. Whatever you do, don't just go to Camping World or your cousin Vinnie who only has experience working on his Jayco toy-hauler.