Yeah i think it would be better to get one to make one for you, unless you feel confident in your own skils, because you get more control over how it looks, not to mention you aren't ripping anyone off. Using someone's work without permission is deeply frowned upon in the art community, with a few exceptions. Even heavy manipulation of an image that was entirely original is still considered ripping.
If you do use it for a forum, I would suggest using one that tiles very well, or using one that fades into a solid color and use css to keep it from tiling. This is because there are alot of different resolutions that people use. Just remember that that best wallpaper can make a page look shitty when it is tiled wrong and stuff.
BP- another thing that would hint at his newbiness would be the use of a "$" in place of an "S"