Icon Search Engine - Download 300,770 Free Icons, PNG Icons, Web Icons
That site has a large variety of Icons if you know how to change them yourself.
The only icons that you can change with LP are the ones on the dock. You can press and hold the icon on the dock to have the "Edit Dock Shortcut" options come up and you'll press "Change icon" and then "Custom icon" will bring you to your gallery. I made a LauncherPro folder in my DCIM folder on my SD card to put all my docks and icons as that way it'll show up in the gallery to pick.