It can be found in Castelia City and Icirrus City.
Somebody gives it to you as a gift.
Castelia City-you can find one in Castelia City, you need to go to the battle company building and head to the top floor and battle the old guy in the corner, after beating him he'll give you an exp. share.
Icirrus City-go into one of the houses and talk to the man show him your basic Pokemon and he'll give you an exp. share.
Hope I helped