Transcripts for some television news broadcasts can be found at Lexis Nexis Academic's TV & Radio News Transcripts.
The Television News Archive from Vanderbilt University can provide a log of the content of network news programs. It does not provide a transcript -- only a listing of the contents of the news broadcast and the time spent on each news segment. Also see the TV News at the Internet Archive.
For additional resources, see the TV & Radio section of the Newspapers & News research guide.