Although I'm not a 501st member yet (still waiting on a few costume pieces), I frequently read the IOC boards. Molli Makes (as BroadSword recommended) seems to be the favorite as far as hats go, but she is moving soon and I'm not sure if you'd get a hat in the time frame you want. You can try Pilot Bay and see if they can expedite your order. I've been told that some of the Disneyland hats can be approved if they have the correct code disc. The Disney hats do run small and aren't made of the most sturdy material, but they are a cost effective option, and, provided it fits may work out until you get another one.