Textures.com - Textures.com is a website that offers digital pictures of all sorts of materials.
Renderpeople - Renderpeople offers scanned human 3D models for visualizations, movies, games, VR & AR.
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.
The Data Visualisation Catalogue - Reference tool for data visualisation
Poliigon - A library of materials, and HDR's for artists including free textures that want photorealism.
Vida - Amazing Visualizations for Your Data