How do you get a Naruto headband?

You can buy it at lots of different anime cosplay stores both online and around the world. They'll very likely have them on hand.

A lot of anime cosplay stores both online and around the world will very easily have them on hand.

Another answer:

Wrong anwer, what he mean asking is the long headband naruto? I could find any they all short 100 cm long. There are hard to find long like naruto 200 cm. sorry