Some non-exhaustive list of textures library:
- the good old and famous https://www.textures.com/
- Substance library https://share.substance3d.com/
- Quixel library Quixel Megascans Library
- for organic characters: https://texturing.xyz/
- high quality open sourced HDRI https://hdrihaven.com/
- same, but for textures https://texturehaven.com/
- the title says it https://cc0textures.com/
In bulk (I don’t go often on these website, can’t guarantee quality): http://texturelib.com/ ; https://www.texturepalace.com/ ; http://texturer.com/ ; https://www.textureking.com/ ; Terminal 26 - Freie Texturen, 3D Modelle und Terrains ; http://www.plaintextures.com/ ; https://www.openfootage.net/ ; http://noemotionhdrs.net/ ; https://www.highqualitytextures.com/textures/ ; Magnet Textures - textures library - www.magnet-textures.com ; https://lostandtaken.com/ ; http://hdrlib.com/ ; https://www.goodtextures.com/ ; https://freetextures.3dtotal.com/ ; Free Texture Downloads ; https://www.environment-textures.com/ ; https://www.cgskies.com/ ; http://bgfons.com/ ; Arroway Textures ; http://www.allcgtextures.com/ ; https://freetextures.3dtotal.com/
Textures search engines: http://textures.one/ ; https://cubebrush.co/
That should be enough 