I was wandering if the source code of the show demo scene of oculus rift scene is available.
Showdown I assume is what you mean?
Yes it is available, look in the learn tab of the launcher.
While you are right that Showdown is available and should work fine on the Rift, I’m guessing that’s not what OP meant.
I don’t have a Rift (yet) but on the screenshot OP posted there’s a button that says “Show Demo Scene”.
That probably fires up this scene : https://developer.oculus.com/documentation/pcsdk/0.5/tasks/ug-tray-start-demo/
That demo scene is not built with unreal.