The best end cap replacement I have seen, not that I have seen many, is at jcferguson's: http://www.airforums.com/forum...ari-15592.html
He did a very nice job all around I think.
If I did an endcap it would probably be in aluminum instead of wood.
The problem with older anythings is that the production was unique and probably not made anymore. The newer stuff is bigger, wider...more expensive.
Have you tried Inland RV? Have you thought about a salvage yard or trailer?
My interest is georgeous rebuilds at birdseed (cheep) prices. But like Wally said, Talk is cheap, Airstreams are not.'