Hi,
OK, now we know the type of door you have from the one picture. If you are interested, I can find the post with pictures of how many have rebuilt that door style. Rebuilding what you have is an option if that works on your end.
Finding a used door made the same as the one for sale, even if used, will be hard. You might find something if a salvage yard deals with old campers for parts. The issue is that it will be old, and that door style was prone to the wood frame inside the door rotting out.
The top corners of the door are square. Is that correct? That makes it even harder to find a supply of them used or in a surplus shop.
Most new campers have radius-top doors, and RV surplus places sell complete door units at very low cost compared to brand new. The surplus doors are new leftovers from the manufacturer.
New square-corner doors do exist for purchase. You get the entire door and frame. To start the hunt for one of these, the door units go by the wall rough opening, not the physical door that swings open. You will have to look inside the camper if there is any trim molding inside that you can lift, or at the bottom of the door, at the floor, to get a width and then a height of the rough opening.
Ebay does have some for sale, and in some cases, they are very cheap as they are new surplus, but again, they need to be the right size, and Ebay's listings change frequently.
I hope this helps.
John