You'll find courtesans in a few places in the Trade Quarter. Oddly, only the ones standing outside will offer you their services. The ones inside the House of the Night won't have anything to do with you.

To sleep with a courtesan, you'll need to pay her 400 orens (or 300 if you have a House of the Night Signet Ring), or you'll need to give her some jewelry, a gem, or a bottle of Perfume.

Note: You can get a House of the Night Signet Ring from Erkyn Blunt in the New Narakort Inn. You can get the formula for Perfume from a brickmaker in the Swamp Forest.