In short

Safe Key Aristocrat's Mansion is a safe key for the building “Aristocrat's Mansion” in Loraine Hills. It opens: small safe. Safe contents are random loot. The key fits only this place.

KeyBuildingDistrictOpens
Safe Key Aristocrat's MansionAristocrat's MansionLoraine HillsSmall safe

Where it is

The building is on the interactive map. All keys, doors and safes of the city: Locked doors, Safes and containers, the key guide.

The game data does not list where this key drops or is sold, so we do not guess.