Question

How do you get your Vivillion to have Sleep Powder?

Answer 1

Move Relearner

Take Vivillion to a Move Relearner.
Dendemille Town in XY and and West of Fallabour Town in ORAS
Give him a Heart Scale and Vivillion will relearn Sleep Powder as Vivillion gets Sleep Powder at lv.1.

Vivillion's Moves Page

Answer 2

You go to the move relearner, and with a heart scale, it should be able to be taught Sleep Powder.

Hope I helped!