To obtain the Morbid Urn, you need to sell the following loot items to any store. Then it will show up in the Bazaar for 250,000 gil.
Phobos Glaze-Reward from the Gil Snapper hunt.
Horakhty's Flame-Reward from the Orthros hunt.
Deimos Clay-Reward from the Trickster hunt.
These items will be rewarded 100% of the time and there are no other bazaar items that are made from them, so there is no way you can miss out on this.