You can find 5.1 music by googling it, then playing it off an external drive or NAS (Network Attached Storage), but it's usually by small artists and honestly there's not a lot of good stuff out there to my knowledge
Soundcloud however supports 5.1 and has some nice content
Netflix supports 5.1 so in the end credits of many movies you get the 5.1 soundtrack. See Where do 5.1 (surround sound) soundtracks end up? for info about soundtracks.
YouTube claims to support 5.1 but does not seem to in my experience for Firefox, Chrome, Hyper, Safari, or Edge, to my knowledge, unless upmixed, as of their move to HTML5 after Flash see https://superuser.com/questions/1044094/5-1-surround-sound-videos-via-firefox-chromeyoutubehtml5-on-windows-10-plays
If you want to see if your browser can play 5.1, start here: https://www.axel-hahn.de/demos/html5-audio/tester-audio-formats.php?lang=en
Windows Media, VLC and iTunes all support 5.1 audio output, but usually that means you have to use a WMA or AC3 file. For a test file see http://www.lynnemusic.com/surround/www_lynnemusic_com_surround_test.ac3
Your receiver can upsample stereo to 5.1 and honestly that sounds pretty darn good for most music. See your receiver's manual for information on how to do this.