If you have the rare(er) open-back model with drop curtains, I don't know of a source to retrofit a door other than the Parker factory.
We regularly have people posting here with questions on removing the rear bulkhead and door, which you can find with a search here in the Projects section.
I replaced my drop curtains within the past week with a new set without windows (as the factory supplies them).
I never run with my canvas down, so by eliminating the windows I get a little better security as well as limiting sun damage.
Here is a shot of my cabin with the center panel rolled up.
Is this what you have?