Gossamer also drops from the necros in Befallen- mostly on the 3rd floor.
And in Unrest, the hags drop it- so 16 is a bit low, unless you are buying
it...

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Unrest is one place I've found a fair bit of gossamer myself, which seems
to
be treasured by the spell casting hags that wander around there. I wouldnt
try to take any from them at level 16 myself however...