[LeoPanthera]

https://musopen.org

https://imslp.org

https://freemusicarchive.org

https://www.jamendo.com

https://ccmixter.org

https://freepd.com

https://incompetech.com

https://audionautix.com

[dualogy]

For flow-compatible electronica, let me add specifically:

https://netlabels.org/electronic-music/

https://files.scene.org/browse/music/

https://netlabelarchive.org/

https://freemusicarchive.org/label/all/