http://www.google.com/images
Not to be a wiseass, but seriously, thats where I find all my wallpapers. I search for "term" dual display wallpaper.
Yep, that works good. I have two computers at work. One with dual, the other with quad, and use the Goog all the time to find good wallpapers. It works great if you type in the resolution that you need.
For example, at my main desk, I have 2 17" monitors, so I need wallpapers that are 2560x1024
(1280x1024 doubled). I just search for that specific resolution, and find all kinds of stuff. Same for my quad. It is 5120x1024.
Also, if you go and search at Deviant Art, they have a ton as well. Again, just search for the specific resolution you are looking for, and you'll find tons.
Out of curiosity, what size are you looking for?