How do I get higher difficulty Plan: Staff of Herding?

So I have made Staff of Herding and I wanted to get the plans for the Nightmarish version, but after killing Izual a couple of times on Nightmare and getting Plan: Staff of Herding two times I don't think that is where I get, but where do I then get it?

To get the Nightmarish, Hellish or Infernal version of Plan: Staff of Herding you will need to go to the last part of Act IV (Prime Evil) on the difficulty you desire and then talk to Gorell the Quartermaster who is standing next to the healer, he will sell it to you:

• 20,000 for Plans: Nightmarish Staff of Herding
• 50,000 for Plans: Hellish Staff of Herding
• 100,000 for Plans: Infernal Staff of Herding

Upgrading the staff at the blacksmith costs:

• 200,000 for Nightmarish Staff of Herding
• 500,000 for Hellish Staff of Herding
• 1,000,000 for Infernal Staff of Herding

Picture is from before the price change.