Moon stones are required to use the Moon Bridges, which teleport you instantly. They can be found on the largest island in the Lost Lake and require you to kill a Moonbeast. These are tough enemies that spawn in groups of two. The easiest way to kill them would be to lure them to the lake and jumping in and killing them with ranged attacks. After you kill them there is a chance that they drop a moon stone. However they DONT always drop them meaning you might have to kill a few of them to get one.