Color photocopies are pretty stable, at least a year, perhaps longer when
light levels are controlled.  The replacements are cheap, too!  The size is
limited to 11x17 though.

Next best would be to find a color service bureau in your area that does
inkjet output.  The process involves scanning the original into a computer
and then printing on a large inkjet printer.  The images will look good, and
protected by a clear laminate last for a couple of years.  Size is limited to
50" by whatever length.  I recently saw an ad for a piece that was 250' long.
 Cost is about $20/sq ft.

Whitney Watson
Exhibitions Designer
Missouri Historical Society