There are only three places: Arborstone mission, Arborstone explorable, and Morostav Trail. Arborstone explorable is your best bet, as there are six stone whatsits in the pillars around each of the four stone bosses.
Anet's developpers : Yeah good idea, 1 stone carving for a gift, but, tadaaaaaaaaaaaaam, we're going to remove the mobs dropping stone carvings to make it harder !!! MOUAHAHAHAHAHAHAHAHA !!!