All Posters is the world's largest online poster store. Not only does this website have a great range (including giclee prints, which last longer than standard posters) the site also has great functionality. You can even upload a photo of the room you are decorating to see how the print will look in it. I will definitely be taking my time to select something. There are so many options!
EDIT: I have just discovered prints.co.nz, which is far more reasonably priced. If you're into kiwiana, it looks especially good.