Skull Mask
In the Lost Forest. Enter the forest and move to the left. Hop on the tree stump while wearing the mask and play Saria's Song. The Skull Kid will then ask Link for the mask.

Spooky Mask
Sell it to the boy in the Graveyard in Kakariko Village.

Bunny Mask
Sell it to the man running around in the Hyrule Field by Lon-Lon Ranch.

Mask of Truth
With this mask Link can talk to the "Gossip Stones" that have the eye on them. They will reveal helpful hints through out the game.