#1 - Check the GigPosters.com Classified Ads
. People are always selling posters in there!
#2 - Contact the designer - on every poster page, on the top-left, is the designer information (website, email). Many designers sell their posters on their websites if you go check it out.
#3 - Ebay
*Gigposters.com does not sell any posters. Please do not contact me looking for posters*