Mouse has some free overland trees, they're more photographic but free to grab. Should be easy for you to mimic, just hand-draw a similar texture, photograph it or scan it, and then change the color setting and merge it then offset it to the right, copy and paste it and offset to the left, use the clone tool to clean it up, then repeat for top/bottom looping, and youl'l have your own variation of this fill that you made. You could repeat that with an ink pen or a pencil, and set the drawing layer underneath a green set to the mode 'color'.