The teleporter stones can be found in act 2 in the little mushroom forest north west of the village. Open the chest straight into the forest, that is partly surrounded by rocks.
On act change the game may be checking if you left a teleporter stone on the ground somewhere, and places it in your inventory if so (one teleporter stone is given to you, the other is on the ground for you to teleport to. Check if you have both teleporter stones (one could be hidden under something or be in the lower part of the inventory); the game could also just check your inventory, and give you the teleporter stones if you do not have them with you.
