Here's a fantastic resource of freely usable texture maps!
Greg Zaal writes:
Joost Vanhoutte has been selling textures and HDRIs on gumroad for a long time now, but yesterday he announced that his site (texture.ninja) is now free :D
It has over 4000 high res textures available for instant download on texture.ninja
If you appreciate his work, he has a patreon page now too.
You are allowed to use these textures for commercial work. Whether you paid for access to the website or if you’re a free user.
You may not redistribute unmodified textures.