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
normal · Lv 22
~1 every 20 kills
Andariel
normal · Lv 12
~1 every 41 kills
The Summoner
normal · Lv 18
~1 every 77 kills
Diablo
normal · Lv 110
~1 every 78 kills
Diablo
normal · Lv 40
~1 every 78 kills
Blood Raven
normal · Lv 10
~1 every 80 kills
DROP SOURCES
expected per kill
Duriel
normal · Lv 22
0.05 per kill
Duriel
nightmare · Lv 55
0.05 per kill
Duriel
hell · Lv 88
0.05 per kill
Andariel
normal · Lv 12
0.02 per kill
Andariel
nightmare · Lv 49
0.02 per kill
Andariel
hell · Lv 75
0.02 per kill
The Summoner
normal · Lv 18
0.01 per kill
Diablo
normal · Lv 40
0.01 per kill
Diablo
normal · Lv 110
0.01 per kill
The Summoner
nightmare · Lv 55
0.01 per kill
Blood Raven
normal · Lv 10
0.01 per kill
The Summoner
hell · Lv 80
0.01 per kill