You might try Vintage Trailer Supply Vintage Trailer Supply | Parts for 1940s-1970s Travel Trailers
but I don't think they carry them, but if you call or email they might have suggestions.
We have a 1963
Bambi and my husband wound up using the old door gasket because he felt he could "revive" it by soaking it in something or other (sorry, not sure what he used...I know he used to use old brake fluid to revive old window rubber on 1930s type cars....) But you might just need to take a good photo of yours, measurements, etc. and go to a refrigerator repair place and see what they have that might work.