There are countless sites and image libraries that contain just textures of all kinds. We have a few of these types of CDs here at work with all sorts of textures like concrete, metal, wood etc.
Usually, they're pulled into Photoshop to colour them up and layer other stuff over them. This is much faster than creating something from scratch (jk).
Besides, this kind of texture when created can be difficult to get right in terms of its randomness and naturalness. Much easier to use something from real life as a starting point.