Creating a realistic texture is very challenging.  Often, when one is needed for a model, I will start by finding an image, usually off Google images.  From there, I will either produce a new texture from scratch, or edit the base image so that it will work properly.  This particular image was edited so that it could tiled without seams.  I also added several details to it.  Here is a demonstration of an edited image so that it can be used as a texture for a 3D model.