Brotherhood scribe robeEdit
Brotherhood scribe robes are worn by Brotherhood of Steel Scribes and are primarily found in the Citadel.
One can be acquired by either killing and looting a scribe or by reverse-pickpocketing better armor onto one of them, then pickpocketing the outfit when they automatically switch to wearing the better armor.
In the add-on Broken Steel, you can get one by waiting for the scribe at Project Purity near the bridge to die from the Enclave attacks.
Outcast scribe robeEdit
The Outcast scribe robe is black instead of red, and only one copy exists.
The only copy is worn by Specialist Olin at the Outcast outpost in the Fallout 3 add-on Operation: Anchorage. You can receive it after completing Operation: Anchorage if you don't save her during the Outcast mutiny, or you can reverse-pickpocket to obtain it.