There are actually two noblewomen that you can seduce, but they both have the same card, and so they're basically the same.

One noblewoman lives in a house (that you can't enter) near Triss in the Trade Quarter, and she'll often watch the jester outside of Triss' house. This noblewoman wears red and white, and when you talk to her, she'll tell you how she likes it "physical and rough." If you give her a Kikimore Claw to prove your manliness, then she'll sleep with you. She won't accept any amount of money.

The other noblewoman hangs out near the bank / town hall in the Trade Quarter. She wears all white. When you talk to her, she'll ask you if you've found her Silk Scarf. If you give her one, then she'll sleep with you.

"A witcher in bed cures my boredom."

Note: You can get Kikimore Claws from kikimore workers once you've read up on them. Kikimore workers spawn in the Trade Quarter at night, and you can also fight them in the Swamp Forest. You can purchase a Silk Scarf from the peddler on Merchant Street in the Temple Quarter.

Another Note: The noblewomen won't always make an appearance. So if you don't see them, just do something else for a while and look for them again later.