Public Domain / Creative Commons / Free
Pixela
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)