I have two sets of camper sliding side windows for sale.  These are the special windows which will accept the mosquito screens, only installed in campers.  You can install them into any vanagon, however there is a difference in window size between the early (to 84) and late (85 up) sliding doors.  I have a set of early windows and a set of late windows.  I have no screens available for sale though.
Email me directly if interested.  The parts are in northern PA.
-Hans