Happy Mask Shop
Ocarina of Time
The Happy Mask Shop appears in Ocarina of Time and is located in Castle Town. It is the basis for the Mask Trading Sidequest, and will only be available after the guard in Kakariko Village receives Zelda's Letter. You can only get the next mask after you sell the first mask. The masks not for sale are all useable by Link after you sell the Keaton Mask, Skull Mask, Spooky Mask, and the Bunny Hood. Once you get the Mask of Truth you can read what the gossip stones say.
Masks for Sale
Masks Not for Sale
Oracle of Ages
A smaller version of the Happy Mask Shop, can be located in Lynna Town in Oracle of Ages. However, masks are unable to be purchased at this particular location.