I take some wallpaper from this site www.wallpapers-hd.net from time to time. It's not exclusively for iPhone but some cool wallpapers have resolution 640x960. Its a cool app. You must try it for once. Hope it will be useful stuff for you.
Also, there are some apps in the App Store. They pretty much let you look through their wallpapers. To get it on your iPhone/iPod, just take a screenshot! That's where my little brother gets his. I prefer to make my own.