I ask you

I want to wallpaper a small wall in my apartment, but I can't. So I want to wallpaper a big flat piece of cheap something that I can hang/tape to the wall.

What is that big flat piece of cheap something?

UPDATE: I've talked to a professional framer and some friends about this and the consensus is to use a foam board, which you can get in large sheets from framing stores or smaller sheets from chain craft stores.