Sounds like you need poster frames. You should be able to pick them up quite cheaply if you hunt around. They usually have a Perspex sheet in them instead of glass, and the edges are clip hinged to make it easier to change the posters without having to remove the frame from the wall.
Edit: I'm on the iPhone so can't post a link, but search for Class Office Supplies. I used to get poster frames from there last year when I was working in a cinema, but they were on the company account so I don't know how much they cost.