Bottomless Box

''A peculiar bottomless wooden box. Its origins are unknown. Some deride it as a symbol of unbrideled avarice. Any number of items can be deposited into the box, and items can be managed while resting at a Bonfire.''

As the name and description says, the Bottomless Box is a box that can hold an unlimited number of items. The items can be retrieved again from any other Bonfire. It's like a pocket-Stockpile Thomas.

However, it cannot store Souls and you can only manage items from Bonfires, not whenever you want to.

Location
It can be purchased from Domhnall of Zena located in the Depths. He can be found sitting between the Boss room and the entrance to Blighttown.