Soulseek probably has the biggest music collection on the internet, and it's a lot better than any website I've ever tried. The client is a little dated but whenever people ask for demos or snippets I just search on there and send them a download link. I learned about it from this sub, a lot of people use it I'm surprised no one's mentioned it.
http://www.slsknet.org/news/node/680
Site says it's a file sharing client but it's used more or less exclusively for music. You'll find all the types of quality you would ever want too, FLAC, MP3, M4A ect.
use Athame...been using it recently for rare/ old dubstep songs that are hard to find. use the link to get free 3 month Tidal (just need a card/ need membership for athame). everything's flac. one of the best program in my life.