When I was making some of my sample instruments for last week's band set I needed some wood grain patterns for the guitar and violin. I created some seamless patterns using Filter Forge filter "Wood" by Kochubey. There are 8 patterns in .asl (with a bevel added) and .pat for PS/PSE with a vertical grain. I then duplicated each pattern and rotated 90 degrees for the horizontal grain. There's a grey one too that you can recolor if none of the included woods are what you are looking for. The patterns are pretty large (3" x 3") so you may want to scale them smaller for smaller elements. And I have also made seamless .png tiles of each of the patterns for the non-PS/PSE users.