Dexter axles can be obtained from any Dexter dealer.
However any Dexter dealer is going to look at you funny if you say I want an axle for my 1966
Dexter doesn't accept orders that way and the dealer doesn't have Airstream axle data. Inland (and a couple of others do.
With that said you don't have Dexter axles if the axles are stock on your '66 Tradewind. The original axles were made by Henschen. A company that is no longer in business.
To order an axle from Dexter you will need to know a lot of data.
Width of axle
Brake drum size
May be spindle size
And I might be missing something as well.
Then there are options like the mount for the shocks that you will have to have welded onto the axle. Change to disc brakes and optional hubs.