The Internet can provide the screenwriter with scripts that sold. While the copyright status of some of these scripts may be doubtful, sometimes the screenwriters themselves are generous enough to help educate you with their own product. John August, who wrote "Go" and "Big Fish", provides us with several drafts of his work.
Once most screenplays find their way online, they spread among the script Web sites like wildfire. The following links should fill your needs.
The best place in the real world to read scripts is the WGA Library at 7000 W 3rd Street on Fairfax in Los Angeles. They have tens of thousands of scripts from shows both current and past.