Lots of royalty-free music at freemusicarchive.org! There are varying licenses depending on the artist and even song, so make sure you look at that before using. Lot of tracks with only attribution/credit required, even for commercial projects.
This is a direct link to a search on there for strictly public domain works! https://freemusicarchive.org/search?adv=1&music-filter-public-domain=1&music-fil...
edit: Downvoted for sharing a site with free public domain music?.. o.O