I think this is a really solid resource for textures: https://www.lotpixel.com
What makes it especially great is that the textures come with full PBR maps, including diffuse, gloss, normal, and reflection. On top of that, they also offer a free desktop import app that lets you import textures directly into Blender with a single click, fully set up and ready to use.
It’s a very convenient option, especially considering that a large portion of the library is available for free.