Where do you get an exp share in Pokemon emerald?

After you have defeated the first Gym Leader, you have to follow a Grunt and beat him. While you are walking back to town you are stopped by a researcher. You will be taken to the president of the company that made the Pokenav. He will ask you to go to find Steven and give him a letter. Steven is deep inside Granite Cave, the cave near Dewford Town. After you have found him, he will give you a TM and leave. Now go to the president and he will give you the Exp. Share.

If you want an additional Exp. Share, use a GameShark.