Here is some info that I found on somewhere on the internet that I modified a bit for myself. Hope it helps.
You need to have three stacks of Nephalem Valor for keys to drop. This is the same for organs, when you're taking out the Ubers in the portals. Playing on the highest difficulty you can manage also increases the odds of a key and organ dropping. I play on
Master I when key farming, and I'm pleased with drop rate.
Key of Destruction
Odeg the Keywarden is in Fields of Misery in Act 1. The fastest way to gain three stacks in Act 1 is to hit up Festering Woods. Its can spawn multiple elites, 1-2 events, and has 2 dungeons with 1 guaranteed elite/champion in each. Odeg always spawns with the
molten effect, and he has a big fireball spell that can do ok damage. Other than that he's pretty easy. Keep in mind that he's a ranged attacker.
Key of Hate
Sokahr the Keywarden spawns in Dahlgur Oasis. The best place imo to farm up stacks is actually the Dalghur Oasis itself. If you find the Keywarden before you have all three stacks, simply run away as he is incredibly slow and cannot catch you - come back when
you've gathered the stacks from elsewhere in the Oasis. As for Sokahr himself, Dahlgur Oasis is quite big and he can spawn anywhere in that zone, so having multiple people helps a lot. Sokahr has an electric attack and a whirlwind attack. When he starts whirlwinding,
I recommend to back up since that attack hurts a lot. Other than that he's pretty easy.
Key of Terror
Xah'Rith the Keywarden is in Stonefort. To gain stacks in Act 3, I teleport to The Keep 3 waypoint and work my way backwards through Keep 3 until I have three stacks, and then I TP to Stonefort. Xah'Rith is the hardest of the three Keywardens as he teleports
around a lot and have a high damage AoE attack. If you are a melee character, it helps a lot if you lure him to one of the many stair cases in the area so he can't run away as easily.
Infernal Machine Blacksmithing Plans
Dropped from Nekarat the Keywarden in Act 4. Luckily, you only have to find this once. Start with The Gardens 2 and work your way through to The Spire. You should have three stacks by the time you get there. This dude can also drop any of the keys, along with
the recipe for the portal device.
Oh wow, I got a key the very first time I've seen a warden on master 2 inferno with 4 players in the game, I thought it was a regular champion, but something was different about this one here, it had no minions and its was very big, like a last standing