Masks can be acquired in the Happy Mask Shop in Hyrule Market. You can only trade Masks as a Young Link. Your reward for trading these masks is the MASK OF TRUTH. With this Mask, you can talk to the Gossip Stones, which, honestly, is worthless. However, some people want to "complete" the game, so here you go:

Get the Keaton Mask from the Happy Mask Shop and give it to the guard outside of Death Moutain Trail.

Get the Skull Mask and give it to the Skull Kid in the Lost Woods.

Get the Spooky Mask and go to the Graveyard during the day. Speak to the kid there and sell him the Mask.

Buy the Bunny Hood and catch up to the Running Man in Hyrule Field. Sell it to him.

When you've sold all of your Masks, head back to the Happy Mask Shop and you'll be given the MASK OF TRUTH.