WHERE TO FARM IT
Which area you farm is decided by the area's monster level, not by the item — the rule is the same for everything, so it lives on the mechanics page.
NAMED TARGETS
Named monsters that can be farmed deliberately. The list spans all three difficulties; each monster appears once, at its most rewarding one. The figure is an estimated average kill count.
Duriel
hell · Lv 88
~1 every 82 kills
Baal
hell · Lv 99
~1 every 113 kills
Diablo
hell · Lv 94
~1 every 114 kills
Mephisto
hell · Lv 87
~1 every 116 kills
The Cow King
hell · Lv 81
~1 every 142 kills
Diablo
nightmare · Lv 110
~1 every 160 kills