Pretty self-explanatory, right? And somewhat overdue - it's not like they were making The Hobbit and somebody said, "Hmm, I reckon this here Gandalf character might become popular with the kids. We should sell costume replicas."
Finally, you can throw out that moldy cardboard one you got for free at Comic Con a few years back and preorder the real deal, an exact replica of the one Ian McKellen wore in An Unexpected Journey. There's just one thing: it costs $219. Lego Orthanc is easier on the wallet, even.
What would you do - ooh ooh - for a Gandalf hat?