Kevin MacLeod’s been providing royalty-free music for probably 20 years: https://incompetech.com/music/royalty-free/music.html
Music
↳ Our family Communities:
➰#Music
Music.world - !music@lemmy.world
Jazz -!jazz@lemmy.world
Album Art Porn - !albumartporn@lemmy.world
Fake Album Covers - !fakealbumcovers@lemm.ee
Obscure Music - !ObscureMusic@lemm.ee
Vinyl and LP's - !vinyl@lemmy.world
Electronic Dance Music - !edm@reddthat.com
60's Music - !60smusic@lemmy.world
70's Music - !70smusic@lemmy.world
80's Music - !80smusic@lemmy.world
90's Music - !90smusic@lemmy.world
Bandwagon.FM lets you filter music by license type.
Archive.org is a great resource for music that has entered into the public domain, if you're looking for some proper classics.
Search the web for ‘creative commons music’ — there are sites specializing in such content, but I haven't used them myself.
Note that some variants of the Creative Commons licenses require attribution, and some prohibit modification of the works or commercial use.