Olivia, I'm a little confused as to what you want… Are you looking for the new-style sliding screen door, the old-style bi-fold screen door, or something else?
If it's the new sliding style, it appears to be custom made for the exact application. You could order it through Scamp (ask about fit with a pre-fire unit) or perhaps you could find a local distributor. It looks like the new style will make a better seal against bugs, but the lower track will tend to catch dirt and it could be expensive to repair if it breaks or is torn.
If it's the bi-fold style you want, I believe it was custom made by Scamp, and they may not sell it anymore. However, member Norm (Honda03842) made one from scratch for his 1991 Scamp 16. His is similar to the OEM unit Scamp used to supply and the design could be adapted to a variety of trailers with curved sides and doors. There's no lower track to keep clear of dirt and it's constructed from off-the-shelf hardware store parts.
Norm's Bi-fold Screen Door
(starts in post #506)