Hey Dreamwarrior,
I actually asked this question a few years ago. They are hard to find as they don't pop up very often. Sleepaway camp had a real small release so apparently not that many posters were printed. According to IMDb it only opened on 85 screens, thats about the only info I can find so I suspect it didn't run very long. Bruce has auctioned a couple advances the last few years but they are larger than the normal one sheets so harder to frame. I think he also had a video one not that long ago. The good news is that I have come across a couple of the normal one sheets so they are out there, you just have to stay patient. I know of one spot where I have seen a couple for sale in the past so I will keep a lookout for you. Hope this helps.
