oneposter.com is my old fave due to just how many they have (and i have some 40 in my room of various sizes hehe) if play.com have started it would also be cool as they have free delivery too., but oneposter.com, also allposteres.com but i dont think they are still up

allposters.com (alternatively, there is www.allposters.co.uk depending on your location) is still up and has a pretty solid selection. If you want artsy stuff, also check out art.com and there's always posters.co.uk which I've found some nice stuff from.