How do you define 'legal'? There's a whole category of music known as 'collage', since it is based entirely on cut-and-pasting. Most of it is only available for download, since they'd never get the rights clearance on the samples.
That said, try this:
www.dictionaraoke.org
www.freespeechforsale.com
and you'll find links to many other similar pages from those.