Public Domain / Creative Commons / Free
PolyHaven
ambientCG
CGBookCase
MaterialX Library (AMD)
3dTextures.me
FreePBR
ShareTextures
3D Jungle
TextureCan
CGAxis (free)
NO Emotion HDRs
Other licenses
The Textures Resource
Quixel MegaScans
Poliigon
Textures.com
Tileable
CGAxis (paid)