Like pretty much everything else in the universe, you can get it on ebay...
http://cgi.ebay.com/TWISTED-BRAIN-HO...QQcmdZViewItem
That's the only one I found of Horror High, but it looks like there's also a sequel (Starring George Clooney no less!) called Return to Horror High. There is a shitload of Return to Horror High for sale on there...
Hope that helps. Enjoy the movie.