I didn't find a single site where you could download buttons or themes. I think all you can do is visit the graphics page on the Self-service where you can download other buttons Here
Or if you have the knowledge you can make your own buttons and them add them to your forum.
Well we have started http://thememotion.forumotion.com as soon as this site gets on its feet we'll be dealing with all graphics requests. I have uploaded a lot of my buttons but you can always request