Where is the principles BSDF shader? I just watched Blender Guru's video on the new-ish Priciples BSDF shader. My current version of Blender is 2.78c. I don't know where to find the release which has Principles BSDF. On the Blender latest releases, they still say 2.78c. What version has Principles, and where can I download it?

Thanks, Phil
It is part of the upcoming 2.79 release and unavailable in previous versions of Blender.

It is not available as a stable release yet and you are recommended to await for the official 2.79 version (available When It's Ready™) to use it, unless you understand the risks of using pre-release testing software.