What? Sure, you can find a t-shirt with a big "3" logo emblazoned on it at Hot Topic, but so can everyone else. For more unique Halo-themed wear, check the developer's website. Yes, your girlfriend will totally wear that Cortana tank top if you ask her nicely.

Where? Bungie Store

How much? $10 to $20 for shirts and hats. $35 to $65 for sweatshirts and jackets.