This article is from the rec.arts.movies.current-films FAQ, by Evelyn C. Leeper email@example.com with numerous contributions by others.
The following is one WEB site i know of:
http://pobox.com/~drew/scripts.htm. There are probably others. There
may be also scripts at sites with archives related to specific films or
sub-genres. Don't forget that most scripts are copyrighted. Scripts
may be obtainable by stores dealing in movie materials or books; see
the rec.arts.books FAQs on bookstores for some suggestions.