Drew's Script-O-Rama is the original site for finding movie screenplays on the web. You need to read good screenplays and unless you live in L.A. you won't have access to libraries of the real thing. Script-O-Rama is the next best thing. Don't buy commercial books of screenplays because they generally aren't correctly formatted.
Simply Scripts, however, seems to currently be the biggest and best site for free screenplays.