The Icirrus city Exp. Share is given to you as a gift by the guy in the Pokemon Fan Club house. The FanClub is directly below the Gym so you shouldn't have too many problems finding it.
The Castelia city Exp. Share is in One of the Large buildings on Main Street (Main Street is the one with all the other streets leading off of it.) Just read all the signs for the buildings until you get to one that says "Battle Company" The manager of the company on the top floor will give you an Exp. Share as a gift if you defeat him in battle. Hope you find a second one and curses to Fling!