Barkus is the Vault City Servant Coordinator, he is responsible for overseeing Servant work throughout the city, their assignments as well as acquisition. All the city's servants, old and new, come through his workplace, and anyone seeking one has to go through him.
Rescue Amanda's husband, Joshua: As with many officials in the post-nuclear world, Barkus is a corrupt person. A hefty bribe allows the Chosen One to ask anything Servant-related and have it done swiftly. That is one of the ways in which Joshua can be set free.
When asking about servants, there's an easter egg dialogue chain only accessible with an Intelligence and Perception of 9 that mentions a scripter named Dan and compares him to the servants in a tongue-in-cheek manner.