If you are looking for the hubcap as opposed to a full wheel cover, you will have a somewhat easier job ahead. The small, center hubcap (that were sometimes used with beauty rings) can be found from a number of sources in chrome plate and if my memory serves correct in polished stainelss steel. A search of the forums for "Hubcap" should turn up the thread that discussed these sources.
The problem may be a bit more challenging to solve if you are searching for a full wheel cover. My '64 Overlander was refitted with 1970s full wheel covers when its split-rim wheels were traded for standard steel trailer wheels in the 1970s. When I needed spares for the two I lost, the search was longer than anticipated. I found that there were two or more variations with the two most common being blue painted center (that seemed to point toward initial installation on an Airstream) or polished center (that seemed to point toward initial installation on an Argosy). I found one of the blue center wheel covers through the Vintage Airstream Club Classified web page
, and two of the polished center variety through and E-Bay auction. So far as I could determine, at that time (about 18-months ago), there were no reproductions available for the original wheel cover.
Another option, that I actually pursued was to obtain a set of wheelcovers to match my tow vehicle. Sometimes this can be an easier and less costly solution. I was able to find a set of four wheel covers to match my '75 Cadillac tow vehicle through an E-Bay auction for about the same cost as one of the Airstream correct wheel covers.
A long-shot possibility would be to check with a tire store that does a significant amount of business with RVers. One that I trade with on a regular basis often has "take-off" wheel covers that the owner's didn't want (preferred plain painted wheels) or when aluminum styled wheels were installed - - I know that with Airstream this would be a very long-shot.
Good luck with your search!