Well go to Castelia City and go to the dock closest to the gate entrance and a guy will say which pokemon u have (the three monkeys) and answer pansage then he will give u a leaf stone.
Route 6 (Show all forms of Deerling in Season Institute) Castelia City
(Answer Pansage) Wellspring Cave (Dustcloud) Chargestone Cave
(Dustcloud) Mistralton Cave (Dustcloud) Twist Mountain (Dustcloud)
Challenger's Cave (Dustcloud) Victory Road (Dustcloud Giant Chasm
(Dustcloud) Shop: Black City-Serebii.net
Hope this helped