The Dark Iron Army

There's an easier/faster way to do this than the guides above.
1. Enter Blackrock Depths.
2. Walk through the gate on the left.
3. Immediately turn right, avoid the fire elemental.
4. Immediately go left, through the door (there's an NPC in front of it).
5. Poison the sack on the left. If you're low level you will aggro, vanish in combat. Avoid the fire elemental patrol.
6. Poison the sack on the right. Again avoid the fire elemental patrol.
7. Go up the ramp completely, poison the sack left from the end of the ramp, avoiding the fire elemental patrol.
8. Poison remaining sacks lying around guarded by 2 dwarfs. For that Sap one, Blind the other, poison the sack, Vanish. Wait for your Blind and Vanish CD to come back, repeat.

Also uploaded some screenshots of the item for you :)

Pretty easy to solo at level 50 if you don't mind repair costs.

Stealth all the way, vanish when you need to, and make sure you're not in the fiery mobs' line of sight.

To get to the grain sacks go into the instance past the friendly guys, then when you get into the large room go for the first passageway on the left.
There's a friendly guy guarding a small door on the right, which is unlocked. Go in and you'll see sacks of grain lying about the place. Find ones that aren't close to the dark iron dwarves and poison them.

When you die now it's only a one second corpse run, unlike before patch 4.0.3a when it used to take ages. Thanks Blizzard.

This mission almost seems to be designed to be solo'd, given a rogues capabilities. And it suits them too. A secret mission behind enemy lines to poison their foodsupplies.

I wish there were more of these missions.