Okay, I have to ask. How would you install it once you found it? The screening is generally double sewn in place all the way around. Considering the camper is 30+ years old, is it worth the effort?
Maybe get a full replacement from Canvas Replacements or similar? At $1000 approx. for full camper that's not cheap, but you might get another 30 years from it if the box and lifter is in good shape.
ETA: maybe check out Bear Creek Canvas. They'll do one end replacement for $350. That's probably near the cost of labor to unsew the old and sew in the new (if you can find the green).
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2012 Chev Traverse LT with TP