One easy way to find more textures is to just do a google search. I found these textures of a bunch of different woods that can be downloaded. http://www.sawdustroad.com/sketchup
One other way is to create your own texture. You can use a photo of the exact thing you are trying to use as a texture such as a gravel path or house siding. This is a little more complicated so I will discuss this in a separate post. Let me know if you have any questions regarding textures.