I can give you a list of many reasons why you shouldn't look for a used one.
1. The door is made of plastic and it gets very brittle (especially the latches) over time. A used one would break and do just the same as the old one did- fall off.
2. The white plastic fades to a yellow over time, so a used door would not match the existing frame. If it was also yellow, chances are the yellows wouldn't match up.
3. A used part like that will be very difficult to find.
4. Removing the existing frame to replace with a new one (and new door) is quite simple. It is only held in with a few screws and putty tape.
The metal doors for this purpose were used on older models and most likely will not fit the opening, or would require major modification to make them work. Also, the model # is irrelevant at this point. The door looks like the top picture above on all 2000 Advancers. Best case scenario would be to take the trailer with you to the dealer, but you could try to unscrew the remaining frame and just take that. That way, you can get the correct dimensions, since there are different sizes. Now that I think about it, it might not be a bad idea to get the model number (RM #), just in case they can cross reference. I just have a funny feeling there is no such way to do that, but in case there is, you will have the number.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR