Here's a concise Whimsyshire guide. I especially like where he maps out the possible locations of Caverns of Frost for the Gibbering gem.
One thing he doesn't say though that I read elsewhere is that the mob Chiltara is hidden underground so he will only pop up if you walk near him, so you might have to walk all over the caverns to trigger him... and that is if he actually spawned...
Edit: I found out you get NV stacks killing elites even in normal level, haha.
Edit2: I am trying to find this guy using my wizard, and here's a rune that I have never ever used: Storm armor, scramble rune. You get +25% movement for 3 seconds after being hit, so I am letting mobs hit me to walk faster. Also the storm armor kills them without me doing anything.
Edit3: More info I found online about this Chiltara - When you find the Caverns of Frost you will be looking for a purple-named, or a rare, boss called Chiltara, that usually spawns in the second level of the cavern. Chiltara does not always spawn, so you may need to clear the dungeon several times. When she spawns, she will drop the Gibbering Gemstone. She does not always drop it, so if she doesn't you would have to leave and try again.
The gem itself is a reference to the hysteria-inducing "Chat Gem" that was present in the Battle.Net lobby in Diablo II. The gem was initially blue, but would turn purple and state that the gem had been "activated" in the chat window above. It had no noticeable effect, leading to rampant fan speculation. A similar gem is located at the bottom of the Diablo III forums.
Edit4: Finally found Chitlara (she was the last item I needed, found others just playing the game luckily). It's definitely worth 150k just for the giggles of this place.
I might also upgrade to hell level so I can farm it, since Whimsyshire drops the same rate as act 4 of the difficulty level that you are on, since hell act 4 mobs has 9% chance to drop iLvl 61 and 1.5% to drop iLvl 62. Not 100% sure on the %s but anyways. Yes, that's how pathetic my guys are...
Edit5: The NM staff costs 400k to craft. I don't have that. Blah.