3 words.
Go to Dalaran.
If they already have a pumpkin head, it will WASTE yours, so don't throw them on people who are already pumpkin'ized. Other than that, You can also do these while mounted (I chased an orc around Dalaran for about 10 mins trying to get a clear shot)
These drop from the headless horseman in Scarlet Monastery: Graveyard.
Have fun!
If you've already done this achievement and still have some Jack O' Laterns SELL THEM! I sold two to a guy for 5g each.
Easy gold :P
The Horseman also drops Tricky Treats, eating three of these in around will cause your character to projectile vomit, getting you another achievement and one step closer to Hallowed Be Thy Name. (Which is also an Iron Maiden song!)