Sorry I missed this thread earlier!
In some of the art stores at WDW - the store outside the animation building at MGM, I think the castle gallery inside Cinderella castle, probably The Art of Disney too... They had a print-on-demand system that had a lot of the posters available. They were printed on art paper and if I remember right ran about $40-$45.
Try emailing email@example.com with your request. They might be able to tell you what is available.