If you're doing videos for youtube, the youtube audio library has some sick tunes and all of them can be used on monetized videos. As for fair use, realistically for an intro none of it will fall under fair use (unless it's an old classical recording where enough time's passed for it to become public domain). Bensound at www.bensound.com also does some good tracks which are free with creditation (literally just put n the description that he composed and performed the track or in your outro) and can be used on monetized videos. Also, you can contact some of the smaller or more independent minded bands. A friend of ours managed to get permission to use an NOFX song on his videos.......incidentally he ended up going a bit mad and now lives on a farm on Israel (true story I swear),pretty certain the two things don't correlate.