Where does this drop?

Im looking for a 20/20 Sundering Horn bow (Rare skin one) Of Fortitude (+30)
15^50%. I'm Poor so dont even try to sell me one. I just want to know

1) What creatures drop them?
and
2) Where is the place that drops gold ones and how often?

Any place where the hornbow drops. There are no certain areas with 15^50% weapons or with any certain mods. That's just luck.