In the Plane of Fear: Enchanter; aside from the occasional Wraithstone, these wisp-like creatures drop no loot.
In Feerrott, the Dream (non-instance): These are found in the vicinity of 865, 110.
In Feerrott, the Dream (raid): Enchanter; part of the raid event "Ritual of Terror" (see this quest entry for information).
In Fear Itself: These worry wraiths hit for a max ~7,500; see invis; and move faster than your standard mob.
In Heart of Fear: The Threshold: Hits for a max ~13,800; sees invis; does not see IVU.