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First off, I see this is your first post -- Welcome!
There a ton of helmet options available on this site from plenty of makers. Are you looking for a completed helmet or a kit to paint on your own?
If you're looking for a complete helmet keep an eye out for them in the "Cargo Hold" as they come up pretty frequently. Also there is a Master Replicas helmet that's available complete that you see pop up on eBay every once in a while.
If you're looking for a kit, Fettpride, Asok, and Bobamaker are the main makers here. Definitely do your research on which helmet you like the most as its a big part of the suit and its cheapest to do right the first time!
Good luck with your search!