The Dragon Hoard Coffer is a treasure that can be obtained from Well of Dragons enemies and heroic encounters. They are used in the weekly quest Reclaiming the Hoard and the repeatable quest Rewards for the Hoard. You can turn in stashes 5 coffers each at the cart next to Elminster.
Dragon Hoard Coffers can be donated to the Tiamat Hoard Reclamation event to affect the fit and improve the rewards gained from facing Tiamat.
You can find this in:
Well of Dragons - Random Drops
Well of Dragons - Heroic Encounters