This item is sold by Wyrmcult Provisioner inside a cavern
The vendor is hostile to the player unless you're wearing an Overseer Disguise buff. The disguise is made by combining 5 Costume Scraps. The Disguise is a one-use item, granting a 3 minute disguise buff. If the buff runs out you will have to farm 5 more scraps to create a new Overseer Disguise.
In order to get Costume Scraps to drop you will need to start a small quest chain that starts at NPC Tree Warden Chawn at the camp of Evergrove in Ruuan Weald, that will eventually lead you to Raven's Wood.
Where usually the item Meeting Note will drop, which starts the questline: