For your shirt, hit up your local guild tailor for the simple to make a Black or White Swashbuckler's Shirt. A good alternative for the ladies who desire the busty wench look is the Azure Silk Vest.
Throw on some Azure Silk Pants and fish up some Buccaneer's Boots from Stormwind Canals for the Alliance or Wailing Caverns for Horde (or the AH.) Then head to the Deadmines and knock off a few Defias until one drops a Defias Rapier and Parrot. Or, you can fly down to Stranglethorn Vale and buy one of two parrots from Nark in Booty Bay.
Finally, for your head, farm 6 Long Elegant Feathers in Feralas from Frayfeather Hippogryphs for a tailored Admiral's Hat or you can settle for a quick and easy Silk Headband, made from simple materials.
Or you can avoid all that shopping and just hit yourself with a spare Hallowed Wand - Pirate from last year's Halloween Event.
Now, your ready to hit the high seas!
EDIT: I missed the easiest costume of all: eating Savory Deviate Delight will transfrom you into a scurvy blight on the high seas (or a ninja). I guess I'll be the one walking the plank this day!