Find (Leoric's Gaiters) in the fireplace of Leoric's Mansion in Act 1: Quest Chapter 1-8-2, with a small chance of refreshing in the fireplace in the room to the right of the entrance.
Find (Liquid Rainbow) in the mysterious chest in the Oasis in Act 2: Quest Chapter 2-6-2, random event in the "Ancient Oasis of Far Far Far Away" area of the map in the second chapter (where the path goes in with the water and the hut), you can get to it in about 20 seconds, or start over if there are no NPCs. When you save the old man, he'll open the door, and one of the treasure chests inside will have a liquid rainbow.
Find (Vetter's Cowbell) in Act 2: Go back to the village after you kill the boss Pillet and there will be a child merchant, buy it right there for 100,000 gold.
Find (Jewel of Balm) in Frost Cave in Act 3: Chapter 3-5-2 of the quest, "Frost Cave" in the map "Killing Fields" in Chapter 3 (randomly appearing in this cave), dropped by the mini-boss Leopardess Chitara (also randomly appearing). (This monster also appears randomly).
Find the design for the Cattle Staff in Act IV: Quest Chapter 4-4-1, which is randomly dropped from the boss Isreal.
Go to the blacksmith to synthesize the Staff of Herding Cows, which costs 50,000 gold.
Nightmare Rank Shepherd's Staff Requires Shepherd's Staff and 200,000 gold.
Hell Grade Shepherd's Staff Requires Nightmare Grade Shepherd's Staff and 500,000 gold.
The Shepherd's Staff of Purgatory requires the Shepherd's Staff of Hell rank and 2,000,000 gold coins.
Transport to the Old Town Ruins in Chapter 1, then head back to the Old Town Road and look for a cow skull on the ground.