Original UI
 

Where can I get the original interface from classic EverQuest? I've looked everywhere for it but I can't find it.

You can't as far as I know. It was something built specifically into the client that has since been removed. I think it might be possible to make a custom one that emulates it with a couple of custom files like icons and such by using /viewport to make the window fit in the middle, but no one has created one that I have seen.