Overview
Keys in Crimson Desert use a universal system. Any generic key works on any locked door (unless the door requires a specific named key from a quest). Keys are consumed on use, so you will want a steady supply.
Where to Buy Keys
Generic keys are sold at Back Alley Shops for 30 coins each. The first Back Alley Shop is just southeast of Hernand City along the stream. A second is located south of St. Halssius's House of Healing, east of Hernand.
Each Back Alley Shop stocks only three keys at a time, and restocking takes roughly two in-game days. Plan ahead if you need many keys at once.
Other Ways to Get Keys
Enemy drops: Bandits, goblins, and other hostile NPCs sometimes drop keys when defeated. It is not guaranteed, but looting every body adds up over time.
World loot: Keys occasionally sit on tables, shelves, and inside containers in buildings and camps.
Alternatives to Keys
Most buildings with locked front doors also have climbable windows. Scaling the wall and entering through a window gets you inside without spending a key. Look for ledges, vines, and windowsills around the building before using a key.
Important Warning
Keys are used automatically when you approach a locked door. This means you can accidentally consume a key on a door you did not intend to open. Be mindful of your proximity to locked doors if you are saving keys for a specific location.