Unlock it, find it in the April Fool Party, or find it in the Festival of Flight!
The are hats with miniature propellers. These types of hats have been around form many years.
It was only available on April fools day last year for about a week.
You cant get a propeller hat on April fools day or from the treasure book from a plush or club penguin toy. Penguin Creamycake4 (member)
You get the hats in April Fools in the Docks place on your map:)