Ammo box, .38 special is a bulk box of 50 .38 special rounds. Upon purchase, the item is removed from the Courier's inventory and replaced with 50 .38 special rounds.
As of patch 18.104.22.168xx, ammo box, .38 special can only be purchased from Chet in Goodsprings. Chet will always have one box available for purchase whenever his inventory is restocked. Note that individual .38 Special rounds can still be purchased from other vendors.