Koroku usually hangs out by the west entrance to the castle when you recruit him, and he starts off with a normal kennel. You can give these to him to give him a more luxurious home, and besides that, it makes your castle look nicer.
Do note that the old kennels can't be thrown away or sold, so keep them in the vault.
||Where to find
||Talk to a man outside of Iku's house in Le Buque.
||Found in the treasure chest in Mountain Path.
||Win it from Billy's card game (2000 potch bet).
||Found in the treasure chest in North Cavern.
||Talk to the lone duck at Alma Kinan.
||Always found the first time you search the corpse near the save state on Mt Hei-Tou.