You're not going to like this answer, but

1. Get your camera,
2. Take a picture of a tree,
3. Make it into a texture.

You're just going to have to search around for stuff on Google. Sorry, but we're not going to do that for you.

Or like I said, make your own.

Either way, you will likely have to make your own .WAD file. (That's a Half-Life texture file.)

To do that, you need this program called Wally:

www.superjer.com/files/wally_155b.exe