Just look up wallpaper stores in your area. If you go to some of the little personal shops you can get BIG samples, they will have a roll open that you can cut a piece from to take home for a look see and try out. Or some big warehouse discounter that sells it either by the foot or by the pound so you don't have to by tons more than you need or want.
The pre-pasted stuff is kinda heavy and you really wouldn't want to be putting water ladened paste on cardboard I don't think, but you could give it a try would be nice if the moisture doesn't cause the cardboard to disintegrate. Non pre-pasted is becoming harder to find, but there should be some available.
Apart from that, might try a "real" art supply store, where they would have bigger (and most lovely) papers, howbeit, way more expensive. But you know the old trick--find it, feel it, nick the name/numbers, then go looking for it cheaply online! (yeah, I know retailers hate us for doing that, but, hey, free enterprise and all that....)
Shylaah