Inca Plastics (http://www.incaplastics.com/page15.html
) used to build most of the tanks for Airstream. I am not sure if they still do, or when they may have stopped, but I would start by checking with them. I was able to get a black tank that was exactly like what was originally installed in my '73. As I recall, their website was not real user friendly, and customer service a bit clunky, but they may be your best bet. They have a catalog of tanks made specifically for Airstream, but if you don't find your tank in there, don't lose hope. Try to find the Airstream part number for your specific tank, and that may be the trick for Inca to find what you need.