.. Where can I get a feather?

Have you seen any feathers?

Feathers usually come from birds.

Check the animal room, through the passage to the right of the sleeping quarters.

The ghost chickens might have feathers.

Why don't you try to get a feather?

Pick up a ghost chicken, and Guybrush will grab a ghost feather for his inventory. (You can also just pick up the ghost feather lying on the ground.)