If you're using Roll20, they have a built-in jukebox feature that can play specific tracks on the different pages of your campaign. The jukebox searches soundcloud for the tracks. I found it a bit clunky to work with, but it does work and is resident in the program so you don't have to run anything outside of it. I found it best to go on to soundcloud and search around for tracks I liked, then go into roll20 and search for that specific track in the jukebox.
But generally speaking for music, I've had the best success with Spotify. Sign up for a free trial of their full membership and they have a huge catalog of music. I'd search for thematic appropriate songs, music from games, movie soundtracks, other playlists users have created, etc.
