Posters of the original Friday are becoming hard to find. If you want to have a sequel or the Freddy vs. Jason poster then they can be found at allposters.com, but the best selection can be found
here.
I have never ordered from this company so cannot vouch for them, but they do have a rather nice original Friday poster, plus a significantly cheaper reprint!
If you are a real fan, then try to get hold of the stand-ups that are available, or figures like
these!