it was added at the factory when trailer was built. Trailer dealer got a quote for me that included it along with the other options like 7' ceiling, Spare tire holder, LED outside rear light ETC. Aluma builds a good trailer. Only thing woudl do different if doing it again is maybe make door taller. Like I said I learned to duck getting back in to unload the hard way LOL.
It is possible to have a door put in your existing trailer at companies that do trailer repair, just have to check & see if better off selling yours & have new built rather than having one installed. For me it turned out better to have new one as old Aluma had good resale.
All Sport | Aluminum Enclosed Sport Trailer makes one, but with angle front lose a lot of storage pace. I did not find great reviews on the brand either, but that was a while back