"they acted like I was from Mars"
Unfortunately, that is something that you will run into from time to time when you tell them the part you are looking for is for a vintage Airstream. I have been told the same thing many times. I guess it is just too much work for them to help you. That is their loss.
Fortunately, there are many other options. You might also try TruckPro on Ackerman RD in SA. They are listed as a Dexter dealer on the Dexter website.
The Rockwell American axles are made in the DFW area, so shipping would be minimal. Quality Trailer has a location on S General McMullen DR. When you go to one of these generic trailer places, tell them you need a rubber torsion axle. When they ask what it goes on tell them "The Space Shuttle".
A torsion axle is a torsion axle no matter what type of trailer it goes under. Picking up the axles in Indiana would also be a good option if going that way.