where can I find one?
After beating the league, go to mr. Pokemon (i think he gives you the togepi egg), he will give you an exp share. Worst case scenario, if you have an exp share in another version, you can always trade.
Hope this helps :)
When you encounter the Red Gyrados after you kill,or catch him/her you will recieve a Red Scale.Take it to Mr Pokemon and he will trade you an Exp. Share.