The Move Reminder lives in the northwestern most house of Fallarbor Town, and will reteach your Pokemon moves for one Heart Scale per move. You give him a Heart Scale, and he'll reteach your Slakoth Yawn
Heart Scales can be found in Routes 104, 105, 106, 109, 115, 118, 124 ,126, 127, 128 and Lilycove City. They also have a 50% chance to held by wild Luvdisc
Move Reminder location
Heart Scale location