The internet is a great place to find posters and prints. When buying a print your primary concerns are the image and size of the page. Online stores are able to include a larger selection of images than local stores. They can ship at media and most report the size of the paper.
This page lists poster stores from Denver or that are of interest to Denver. I;ve added a few select images of Denver to the Poster Store. You will find a large number of images from Colorado on the photo page.
Red Army Art (www.redarmyart.com) This Denver store sells items with propaganda images from the Soviet empire.