I assume you mean the hammer for the event with Kayelinth?
The lost and found center is in the entrance hall of your castle, when you enter it from the main gate near the Aldlyn city board.  It's the room where the doorman stands near the entrance to the throne room.  Or when you use the teleport stone and are at the sleeping chambers, you need to go down the stairs to the throne room and follow the path down in the bottom middle.
The guard you need to talk to is on the right near a table on that map.