It does indeed drop in sunwell plateau. It's actually a trash drop although some mods like atlas loot still had it listed as a felmyst drop the last time I looked.

Doesn't seem any more rare than any of the epic items that drop from black temple or mount hyjal trash. 2 dropped already for my guild and it's not like we cleared trash that many times as most times we have wipe recovery in place... means we only have to  clear about 1 trash pack nearest to the boss we are currently fighting every 2 hours or so.