It costs 100 and is available for purchase on every Tuesday excluding the Winter months from the Casteliacone Stall on Mode Street in Castelia City.
In Pokémon Black 2 and White 2, it is available for purchase once every day. The player character can chose to buy one or twelve at a time. There is an Item Manic in a trailer east of Nimbasa City who will buy them for 2,000 Pen (Pokémon Yen) each, making them a highly profitable item.
In Pokémon Sun and Moon, the guy at one of the Pokémon Centre Cafe will give you a Casteliacone if you are lucky enough.