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 45 kills
Baal
nightmare · Lv 75
~1 every 64 kills
Diablo
nightmare · Lv 62
~1 every 65 kills
Mephisto
hell · Lv 87
~1 every 65 kills
Diablo
nightmare · Lv 110
~1 every 81 kills
The Cow King
hell · Lv 81
~1 every 91 kills