Completely subjective list of places with good quality assets (graphics, sounds) in formats useful with Castle Game Engine. See and post on the #assets channel of our Discord for more.
Low-poly 3D game assets, mostly free and public domain. See also on KayKit page on Itch.io.
Many 3D model packs, free to use for both personal and commercial projects.
3D models, in glTF format, uploaded by various artists. Some free (on clear licenses, like Creative Commons), some paid.
Note: you can also import from Sketchfab directly in CGE editor!