Changing out the existing door for a complete new unit would be the "ideal situation" finding a used (or possibly new) screen door would work provided you can find the correct one to fit your door unit. Both of those are going to be either expensive or time consuming.
You might want to consider one of the magnetic screen door systems that have been popular to install in frequently used doorways. Here's one such option that costs less than $10:
http://www.amazon.com/Docooler-Hands...n+door+curtain
We use a similar product on our garage doors during the summer to keep bugs, especially gnats and mosquitoes out of the garage when working on cars, ATV's or other equipment in the garage.
It's been my experience with older door units that trying to rebuild or replace a screen door in the RV door unit that they seldom fit or function properly after replacement. It is very difficult to find the "original factory screen door" and aftermarket doors often just don't fit properly. I've had better luck just replacing the entire door unit, both the solid door and the screen door as a unit. That does get expensive, so it may not be an option for you. In the interim, just using the above magnetic screen door will get you by until you can find the right way for you to proceed.
Good luck.