Once you learn that Hisui might have the info about the dealer, you need to search for her. She is available in places where food is served, such as INN of the west sector, the outdoor cafe in the eastern sector, 2nd floor mansion dining hall, the open area south of the mansion, eastern bazaar, and the tavern near the docks. Alternate between these locations and the brothel, to progress her story. If you don't find her more than once in the same day, skip a day and search for her again. Eventually you'll find her at the docks being assaulted since she has been eating in the tavern without paying money, prompting you to clear her debt, after which you'll get the dealer info saying that he's in the slums. Then talk with the gang boss in the mansion for the permission to access the slums.
