I would have to agree with the vacuum form route for getting an accurate EVA-style helm.... With those shapes, there simply isnt anything close enough to them that you can buy.
Helms like the ones the Spartan III's use are this way; theres no way to mimic that ridge in the top-middle without just vacuum forming one:
EVA helm is the same way:
So, GL with those vacuum forming !
It isn't too hard though....