OpenGameArt.org has plenty of stuff under a range of open culture licences. Kenney's assets are pretty popular these days - there's a lot of CC0 licenced stuff on his website.
Sketchfab apparently has a lot of Creative Commons has licenced models (you'll need to use search filters to limit the results to particular licences).
In all cases, make sure you're respecting licence terms (some licences require attribution while others don't, some allow modification while others don't, etc.)!