[Vulture]

There's no guaranteed location to get Demetrius. If you want to know where and how to get it in your game you can download the Save Game Scanner from the mod section. It's easy to use and will tell you if there is a Demetrius in your current save game and if yes, where it's being sold/to be found.

€dit: you can use it on any saved game. Even if you're still in the training cellar. Treasure and vendor inventory are generated the second you start a new game.