This item drops randomly in Tanaris from
- Wastewander Thieves and Bandits
- Southsea Pirates, Freebooters and Swashbucklers
- Sandfury Shadowhunters, Blood Drinkers, Witch Doctors, and Shadowcasters (in Zul'Farrak)
- Searing Roc
The drop rate is reported as between 0.5% and 1%. As is common for items that start quests, the real drop rate is somewhat higher, since while you have one of these - or have finished the quest it provides - it will not drop for you again.
People attempting to farm for this would be well advised to join a Zul'Farrak run, as a clear of ZF will usually drop this item.