Cistern in The Ragged Flagon in Riften, appears on a shelf behind the desk after completing 45 Thieves Guild jobs. When using Destroy the Thieves Guild, it will instead be in Sadri's Used Wares in Windhelm, where it can be either bought or stolen.

Normal Location

The Ragged Flagon in Riften

Destroy the Thieves Guild Location

Sadri's Used Wares in Windhelm