After completing the Dawnguard main questline, she can normally be found at these same locations, although you will occasionally randomly encounter her in the wilds of Skyrim, where she can be recruited to follow you again. After completing Beyond Death, she can also be found in Valerica's study.
Her main location during gameplay depends on which faction you ally yourself with-if siding with the Dawnguard, she can be found at Fort Dawnguard if siding with the Volkihar vampires, she can be found at Volkihar Keep. Kindred Judgment: Confront Lord Harkon.Touching the Sky: Find and retrieve Auriel's Bow.Unseen Visions: Perform a ceremony to read the Elder Scroll.Beyond Death: Search for Valerica in the Soul Cairn.