Our Coca-Cola bottler also bottles Dr Pepper, so you're not going to find it except for at a few soda fountains.

Or if you want to be super-technical about it you'd need a time machine, because Mr Pibb was replaced with Pibb Xtra about twenty years ago.

You could just buy some Dr. Thunder and mix some dirt and cigarette ashes in there, pretty sure it'd taste the same as Pibb :P I'm just teasing, Pibb is fine.