Well, there are too many woman with a slave collar in Fallout 3. If your referring to a companion, that is Clover. You can find her at Paradise Falls. You have to have bad karma to buy her, but if you fire her and rehire her again, you don't need any karma requirements. You have to buy her from Eulogy Jones for 1000 caps, 500 if you've got a high barter skill. If you don't know where Paradise Falls is, it's north of Vault 101; far north.