Carried by Maven Black-Briar in Riften. It must be pickpocketed from her. If you have made her Jarl, she will be in Mistveil Keep. If she isn't Jarl, she will be roaming Riften, and can often be found in Black-Briar Manor, Black-Briar Meadery, The Bee and Barb or the marketplace.
"While Maven Black-Briar never goes anywhere without a personal bodyguard, she knows that a woman of her station has to be able to defend herself as well."