I’m always on the look out for new wallpapers. Unfortunately, a large portion of wallpapers out there aren’t the right aspect ratio to fit my widescreen monitor. One of the best resources out there for finding images is Flickr. A quick tip about finding a wallpaper to fit your monitor is adding your monitor’s resolution to your flickr search. If you’re looking for Macs, and your monitor’s resolution is 1680 x 1050, try searching for “Mac 1680×1050.”
It won’t always help you find what you’re looking for, but I have found quite a few wallpapers using this method.