Gelderns is outside the southern exit. You'll see a group of rangerish looking people just outside the gate. Nearby to them there is a chest on the ground with a rock near it. The stone is on that rock.
There may be more than one stone though. I've found two for Okara.
There seems to be multiple stones for each town except Montera, for which no stone has been found.
Silden: North of town, at the stone circle. Geldern: South of town, at the camp site with Boris. Trelis: In the cellar below the temple in town. Montera: No known teleporter stone. Gotha: In the smithy or In a chest at the entrance in Gotha Okara: On a rock left of the entrance Vengard: In the cave in Gotha. Faring: In the collapsed mine at the town entrance. Ardea: In Conz house, or on the lower roof of the Lighthouse. Reddock: Within a chest in the cave with Golems. Cape Dun: In the Warehouse. Nemora: Within a chest, at the right side of the North entrance.