Look in the yellow pages for a map shop. They probably have contraptions available for displaying maps, since that is their entire business - selling maps, and then having their customers display them.
Besides ordinary people, their customers for especially large maps might be international businesses, universities, museums, all of which would need display capabilities for their maps, just as you do.
Just a thought.
(There's a map shop where I live, dealing exclusively in maps, so I know they exist.