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It should not be super difficult to find a door from a donor vintage trailer, but it will likely be a search that requires a lot of patience. Being that you are in the UK, it will get expensive. Seems like the going rate for used vintage doors used to be somewhere in the $600 range. Shipping would likely be hundreds as well.
I have rebuilt my door, and although it was no task for the casual DIYer, it wasn't rocket science. I built wooden jigs that matched the curvature of the shell's door jamb, then removed the skins from the inside and outside of my door, and fitted it to the jig, then worked it until it maintained the shape. I then sourced replacement aluminum sheets and riveted them back into place with the door clamped to the jig to ensure the shape was maintained. Getting the accurate curvature is key.