Here's a collection of sites where you get materials, textures, skyboxes, HDRIs, and the like!
http://texturer.com/sub72/
Free textures that can be used for personal or commercial projects.
Open Game Art
A collect of textures for games released under various licenses such as creative commons.
https://textures.pixel-furnace.com/
Free textures for non-commercial and commercial projects. Credits are not required but appreciated.
https://www.clipartmax.com/
A collection of transparent pngs that can be used in game projects, but it seems to be mostly copyrighted stuff, so I strongly recommend only using these for free stuff like mods, fan games, etc
https://www.renderhub.com/seamless-textures
Paid site where you can purchase textures and materials.
https://www.textures-resource.com/
A site that collect textures ripped from video games. Not usable in commercial projects, but you may be able to use these for mods, fan games, etc. Site also features skyboxes from games!
https://www.textures.com/
A site providing both paid and free textures for your projects.
https://3dtextures.me/
Free Creative Commons 0 textures you can use for your projects.
https://polyhaven.com/
Free textures that can be used for personal or commercial projects. Site also features HDRIs which may also be usable as skyboxes!
https://ambientcg.com/
Free textures and HDRIs!
CG Trader
Kind of a confusing/messy interface on this site, but they do have free textures!
https://www.cgbookcase.com/textures/
free creative commons 0 textures.
https://www.sketchuptextureclub.com/
Royalty free textures that can be used for both personal and commercial projects.
https://cgaxis.com/all-access-v5/
Site providing both free and paid textures. Registration is required to download the free stuff.
https://quixel.com/megascans/free
A collection of free textures and models, seems to require downloaded software and/or an installation of unreal engine.
Fab
Related to Quixel, another source for free textures to use in unreal engine.
https://texture.ninja/
a collection of public domain textures.
https://itch.io/game-assets/free/tag-textures
Lots of free and paid textures are available on this great site!