Ccmixter.org have a ton. Also soundcloud. Aside that the usual ways like Youtube.Also keep an eye for remix contests. Plenty of good vocals from a few well know projects/bands for free ,a chance to get featured and earn some $ or other prizes. Source: over 1 year ago

Find some fitting music to use in your videos. Source: almost 2 years ago

Creative commons remix community (some are non-commercial): ccmixter.org/. Source: almost 2 years ago

For Free and non-copyrighted samples: CCmixter and Freesound. Lots of good stuff there. Source: about 2 years ago

Http://ccmixter.org/ tons of music with a variety of creative commons licenses. Source: about 3 years ago

Epidemicsound.com, artlist.io, audiio.com all have some decent/good quality music but they are all subscription based. It's ROYALTY free, but the license to USE isn't free. Be careful if you go with any subscription, with some of them, the license is only good if your subscription lapses but with others, the license is lifetime regardless of your subscription status. Source: over 1 year ago

Just in case you were thinking about buying it audiio.com sfx lifetime licence is pretty bad. After being blasted with ads for a one-off lifetime payment for sound effects I bit the bullet and purchased. Turns out it's pretty terrible. They have virtually nothing beyond a few very short city and landscape sounds and a couple of whooshes and some random effects. Source: almost 3 years ago

Check out https://audiio.com - it's fairly newish but they're updating weekly and they have a lifetime membership for a reasonable cost, the music is decent, and the licenses cover virtually anything you need it for. Source: about 3 years ago