Look at RV Shows..
If you go to local RV show or dealers with lots of inventory, the stickers from the manufacturers are still on the shower stalls that are installed as complete units. I think Indiana Fiberglass is one of the larger makers, including the circular units. It is unlikely, though, that a rectangular unit will work with curved side walls in Airstream well, so you may have to start with shower pan from Home Depot or Lowes, and work with aluminum sheet on side walls to make your own... I've never seen one made of aluminum...
In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...