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
nightmare · Lv 55
~1 every 46 kills
The Cow King
nightmare · Lv 61
~1 every 63 kills
Baal
normal · Lv 60
~1 every 66 kills
Mephisto
normal · Lv 26
~1 every 66 kills
Diablo
normal · Lv 110
~1 every 66 kills
Diablo
normal · Lv 40
~1 every 66 kills