All in the question really.
In the house east of the Pokémon Center, the researcher will give you an Eviolite if you have enough pokémon in your Coastal Kalos Pokédex. The Eviolite raises Defense and Sp. Def, but only if the pokémon holding it has not reached its final evolutionary form.
There.
Source.
You can get the Evolite by talking to the scientist in shalour city near the Pokemon center. Go to shalour city, go inside the house next to the Pokemon center, talk to the scientist and he'll give you it. He'll only give you it if you have enough Pokemon in your coastal Pokedex.
Source:http://m.youtube.com/watch?v=N13jqAMpRpY&desktop_uri=/watch?v=N13jqAMpRpY
Hope I helped!
Fly to shaliour city, and take to the left. Talk to the scientist closesr to the door and he will give you an eviolite.
Hope I helped:D Source