Barman (Rat Pits)

The Barman is located in the Rat Pits in Port Sarim and Varrock, and sells stew for 20 coins or beer for 2 coins if talked to.

As he's located in the Rat Pits, you must have reached a certain stage during the quest Ratcatchers, more specifically in the part after you help Smokin' Joe, to talk to him.