They do show up on Ebay, in various sizes. They were common on many different trailers in the 1950's and in many sizes. I have four different sizes on my Liner, not counting the front and rear windows.
Keep looking, I have found replacements.
Bill Kerfoot, WBCCI/VAC/CAC/El Camino Real Unit #5223
Just my personal opinion
1973 Dodge W200 PowerWagon, 1977 Lincoln Continental, 2014 Dodge Durango
1979 23' Safari, and 1954 29' Double Door Liner Orange, CA