In the Hissing Wastes you can find a being that resembles a Chantry sister, found in seemingly random places of the area. Speak to her four times (there are no dialogue options). The last time you speak to her, she will make a comment about the Inquisitor finding them, saying, "Take this gift. Do not falter. In the absence of light, shadows thrive." After this she disappears, leaving behind some loot including the bow.
The Chantry sister seems to appear most often somewhat near camp areas.
By finding her once, saving after speaking to her, then loading the save again, she will be available for conversation once more. Doing so repeatedly counts as finding her several times and will give you the bow.