There are a total of four masks you will need to find owners for. Four other masks are also available for you. Information about each mask and their selling locations are given below.
Keaton Mask - Sell this mouse-like mask to the guard in front of the entrance to Death Mountain.
Skull Mask - Sell this mask to the lone Skull Kid in the Lost Woods. To find him, from the entrance of the lost woods, head left.
Spooky Mask - Give this wooden mask to the child in Kakariko's graveyard. The child only appears when the sun is up.
Bunny Hood - Not exactly a mask, but if you find the guy running around Hyrule Field and sell it to him, you will receive a generous amount for the hood.
Other Masks
These masks you can borrow from the mask shop only after you've sold all the other masks.
Mask of Truth - This is one of the special masks. If you wear this in front of a gossip stone, talk to it, and it will think you are also a gossip stone - these stones will share gossip with you!
Goron Mask - These special masks will make you look like a goron! Wear the mask in front of a goron and hear what he has to say.
Gerudo Mask - This mask will make you look like... a girl? Wear the mask in front of people to make them think you are a Gerudo.
Zora Mask - Wear this mask and you'll look just like a Zora. Talk to the Zora's and hear what they'll say about it...