The Lockbox Key artifact can be found in Tol Crossing, in the north-eastern part of Sea of Núrnen.
Lockbox Key, Third Age, circa 2930
This key was crafted long ago by the locksmiths of Khazad-Dûm. It is able to open a wide array of locks. It would be of great value to a burglar or spy.
"The Orcs are making things mighty difficult. This might be my last run in a long while.
My wife and I… we need to… be… elsewhere.
The lockbox. Open it. Now.
These are hard times, and I need to know you can pay. Up front.
Well, now. That's hardly enough for both.
We've always… that's all I have! Please, my wife!
Should've saved more. Now: Room for one on the boat. You decide…"