Keaton Mask: Sell to the guard standing at the entrance to the Death Mountain Trail in Kakariko Village.
Skull Mask: Sell to the Skull Kid dancing on a log in the Lost Woods. (When you enter the Lost Woods from Kokiri Forest, go left.)
Spooky Mask: Sell to the kid imitating Dampé in the graveyard during day-time.
Bunny Hood: Sell to the jogging man jogging in Hyrule Field. He is found typically near Lon Lon Ranch. He is not easy to find but don't give up.
Since you've sold all the masks and given the Happy Mask Shop owner the money, he will give you the opportunity of taking some new masks; the Mask of Truth, Gerudo Mask, Zora Mask, and the Goron Mask.