Where is the exp share in Pokemon platinum?

In Eterna City, go to the building that starts the biker route. Talk to the dude in the white coat, with black hair and glasses. He'll say something like, "Oh hello! You probably don't remember me, but I am Professor Rowan's assistant and Lucas/Dawn's dad! He told me to give this to you." he hands you the exp. share. Hope this helps, good luck on your Pokemon journey.