6 Awesome Sites for Free Blender Textures
300,000 Free Creative Commons 3D Models Available on Sketchfab
Credits — alpha.wellcomelibrary.org
Personally I think the highlights are probably the leaves' category together with the more recently added fabrics that has excellent quality.
Except that it never actually gets around to rendering anything.
So, sorry, there's no plans to ask for external creative involvement.