I got my new fridge from a local Camping World when it went on sale.
I would suggest just keeping an eye out for a sale at the local rv places around you.
You can check the dometic website for replacements based on size. That way you will know what your looking for.
Dometic Replacement Chart
I found my Camping World did not want to work on my vintage trailer. Don't know that I would trust them to anyway.
If you cannot do it yourself, I would recommend that you look for an Airstream dealer familar with vintage units to do the work.
It's not that they are any different, it just seems like some of the RV places only want to work on newer units.
Good luck with your replacement.