Hi, and welcome to the forums!
I'm not sure if this part is still available or not. There's a few vintage parts places online and it seems like the really obscure and rare stuff is available from Inland RV (google it). Their website doesn't show everything that they stock, so be sure to call and inquire.
You can also post a wanted ad here: - Airstream Trailer Classifieds - Airstreams Trailers For Sale
. You can log in with your Airforums username and password. Alternatively, you can subscribe to the "parts for sale" section of the same site. If you set it up as such, you will receive an email anytime a new ad is posted and you can then decide if you want to contact the seller. Often there's people selling used parts due to a remodel or damage to their Airstream.