For original stuff try http://www.nostalgia.com.
Some of the stuff is pretty pricey but all the posters are originals in most cases. I've used them for the withdrawn Spider-Man (World Trade Centre in his eyes) poster and it came very well packaged. Also try http://www.allposters.com
which sells reproduction stuff. http://www.ebay.co.uk
is also good. Although be weary of people saying the stuff is original. Not always true!!!!!!