This is, by far, the rarest out of the three elemental instruments. Our guild has done over 15 runs on the Fennin script and we've yet to see one drop.
Ranked in availability:
1) War Drums of the Rathe (Quite common from the Stone ring in PoE)
2) Lute of Flowing Waters (Killed Griolin 3 times, dropped 2 lutes)
3) Blaring Horn of Fire (Killed Javonn, Azobian, Hebabbilys and Reaxnous so many times, and nothing)