The red has a 1% drop rate, but the other three are 13-15%.
They're all in The Temple of Ahn'Qiraj, which has no prerequisites. As long as you're in a raid group, you can stroll right on in.
There are two instances. When you hit The Scarab Wall, you'll see one straight ahead to the left. Don't go to that one. You want to go to your right up the winding path to the top of the hill.
Group or No Group?
Some classes can solo stuff. Some can't. You decide. As a rogue, I needed a healer and husband's druid sufficed. My main issue was keeping threat, but I got 3 down without any poisons on my daggers because I always forget poisons, so if you have just a little gear and a healer you should be fine.
Video of Walkthroughness
The following video is not by me. I make no claims to this vid.
The strat in the video is thus: get to the four Sentinels shown (you enter and they're right in front of you), kill three of them, run out of the raid, and they'll reset as four -- leaving the bodies of the fallen. The vid itself has more specific tips, but there ya go. You just need to leave 1 or more alive and you can keep farming them over and over.