I was running a few monitors at once a while back, all of the wallpapers i found tended to lose a bit of their charm thanks to the gaps between the screens. In the end i went with one of the wallpapers that i had on my single monitor that tiled across to the others, so they all had the same image, but it still flowed together. But that's just personal opinion.
Anyway, if you're after wallpapers of any description check out http://wallbase.net/. Never lets me down