Kevin MacLeod’s been providing royalty-free music for probably 20 years: https://incompetech.com/music/royalty-free/music.html

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.