A little while ago I was doing the same thing you are. At first I thought of buying the Level 1 poster pack, which comes with like 5-10 movie posters and some stickers. It said it was only like $6.95 which seemed like a good deal, but shipping costs like $12, so I decided against it.
So then I was browsing around the infamous searchbar on NS and stumbled across this site (which is apparently a good 5 years old) called Rasterbator.
It lets you make your own posters of whatever picture you want. Personally, I did my entire dorm wall with one picture and it came out amazing. For best picture quality, use 1mm dot size, and try to print on card stock paper. It rapes your ink cartridges so try to print them out at your school if you can. Hope this helps.