tmbh on 13 January 2008
http://workbench-music.com/music/
Some of the albums are 100% free to download but you may need to get the author/composers permission to use for your documentary. Contact the artist directly via the site and see what they say.
Failing that google "royalty free music for broadcast" or something similar
http://www.premiumbeat.com/
http://www.proudmusiclibrary.com/en/search/
http://freeplaymusic.com/
Then make your documentary ensuring I get a credit!!
