formerly CGTextures; most heavily used photosource library in the world until 2010; still offers a generous free account tier for medium resolution textures (x512, x1024)
Prototyping blockout textures
When making the blockout, we recommend using gridded placeholder textures to prototype the level geometry. This helps establish consistent scale and metrics for the level.
A skybox is a special type of 6-sided cubemap texture that gives the illusion of a distant sky background. Contemporary game engines usually use procedural sky shaders / skydomes, but sometimes the old fashioned ways are more effective.