Welcome to the forum!
I would steer clear of Shepperton Design Studios, long story but ethics play a big part in why to avoid.
I have recently purchased a helmet from RS Props, their Stormtrooper helmet is cast directly from a helmet used in A New Hope. The only issue you might have is time! I would imagine a ready made completed helmet will be loonger than the few weeks left before christmas.http://www.rspropmasters.com/
Alternative is the recently released Black Series Helemt from Hasbro, think this is available in Toys R Us, Amazon etc....
Hope this helps