I need to mega evolve my aerodactyl!!
You get it at the Ambrette town fossil lab after you get your sail/jaw fossil, the scientist on the right gives it to you.
Source- THIS and the time I got my own aerodactylite in X. :3
You get the aerodactylite once you show the scientist in the same place to turn fossil to Pokemon you have it - the jaw/sail fossil. This is located in "Ambrette Town".
Watch yourself: https://www.youtube.com/watch?v=f2EGZy6FUvs
Hope I helped! :)