Someone on the keira_daily
community on livejournal said that the Movie Idols poster was a freebie in the latest issue of Film Review
magazine. It should be pretty easy to find in the UK, and in the US it can be bought at some Borders stores.
ETA: If you don't have any luck finding it in a store, you can order copies of specific issues from their website at http://www.visimag.com/filmreview/