I have seen a few threads on these forums. XDA probably has a decent selection too. But my favorite spot to go for one is the LP forums. LauncherPro Forum • View forum - Theming and Customization Check out the share your dock backgrounds thread mainly, but some of the others are pretty good too. I know it is 140+ pages...but it is full of good stuff.
The way LP works it's themes is a bit different than the way ADW does.
Side note, these will work on any ROM. You don't need to be rooted either to theme LP or ADW for that matter.