Philippa Eilhart maintains residences in both Vergen and Loc Muinne. In Vergen, she has her quarters from which she operates, and in Loc Muinne she keeps a house. Her Vergeni quarters are also shared with her student, a leashed sorceress named Cynthia.
Geralt visits her place on several occasions so it is impossible to miss. However, the easiest way to navigate to here is to find Haggard by the Smithy and head for the stairs at the end of which are her quarter's red doors.