Use peer to peer sharing softwares, but i wont recommend it. Since they can be traced.
Prolly Torrents, there the safest around.
Search on youtube for tracks.
Depends really, if its something fast paced, maybe some drum n bass, dance etc...
If its slow, some chill out music, trance hiphop whatever man.
Ideally i wouldnt use my favourite songs over drift videos, coz they just wont fit in. You end up hating that favourite song then.
Or keep watching other peoples LFS videos and get some idea to inspire you!.
I've used some FLV converters, they make the track quality so lame when you use it over your video. It just sounds so nasty. Use a high quality track.