Unity Asset Store - Unity's asset marketplace for game makers.
Textures.com - Textures.com is a website that offers digital pictures of all sorts of materials.
Kenney Assets - We've created over 40,000 images, audio files and 3D models for you to use in your projects. Thanks to our public domain license you're even allowed to use them in commercial projects!
Poly Haven - Poly Haven (formerly known as Texture Haven) is an online website where you will find scanned textures without any cost and that are of high quality.
GameDev Market - GameDev Market is a marketplace for high quality, affordable 2D, 3D, GUI & Audio game assets, handcrafted by talented creators around the world.
Poliigon - A library of materials, and HDR's for artists including free textures that want photorealism.