For the beginner looking to get some nifty textures into the game and start experimenting with kgPanels, I suggest a couple of downloads that can help you get started.
Anything made for Btex (If you just want the bottom paneling, I highly suggest Btex as a substitute for kgPanels.)
You can also find custom artwork in many down-loadable UI compilations. Don’t worry, if you aren’t interested in the entire compilation the textures are pretty easy to find! Generally you will find the .tga extension files either directly in the interface folder OR in Interface/[non-addon name] folder. A little digging can yield some nice work–just remember that if you have plans to publicly share your own UI compilation to give credit back to those whom you borrowed from!
There is a lot of content out there labeled as “Media Pack.” These are generally for shared media (textures for buttons, etc.) and not for use with kgPanels.