I like the ones on http://www.mandolux.com/ (esp when using ultramon), but there are numerous sites out there that can be found with a basic google search.
i see at that site you have a picture for the right and left monitor, how do you place a pictrue in the background for each??? i really like that site but i dont know how to put a picture in the background on one wiht out doing the other...
I run two monitors right now, both 1280x1024. There are very few dualscreen wallpapers, if you want two different ones on each screen what i did was attach two 1280x1024 wallpapers which made it like 2560x1024 (in microsoft paint which was a pain) and i set the desktop wallpaper to "tile" mode.
Using Ultramon, or DisplayFusion (free), you can have different wallpapers for each monitor. Otherwise you will need to do as above, and combine the images into 1 big image.
Some other sites I found w/ free wallpapers from a google search:
Finding multi-display wallpapers is pretty easy. The biggest problem I am having is finding them in high resolution for my dual 24" monitors (1920x1200 * 2 = 3840x1200) here at work.