Sorry if this is too late to be of help. Just found this thread while searching for another and thought I'd try to help since it's only a month old.
If you want to follow the semi-techno samurai style of
GWs official Mk1 armor, there's this small line:
https://bitsofwar.com/search?controller=search&orderby=position&orderway=desc&search_query=samurai
These look almost exactly like what you want and come in both with and without gas masks covering the lower half of the face:
Without (closest to what you described):
https://artelw.com/Male-Heads-with-helmets-p147658862
With (make for some nice variation):
https://artelw.com/Helmeted-Heads-with-Hazmat-Masks-p119875829
If you're looking for more semi-modern scifi tactical style heads, you can take a look here:
https://anvilindustry.co.uk/heads-helmets-crests#/pageSize=6&orderBy=0
Specifically the Black Ops Helmets seem to match your description, but the rest are close too. They also have a set of Top Knots if you want to make them really look like the official
GW design.
These look a lot like Halo ODST helmets if you want some of that look going on:
https://zealotminiatures.com/zealot-shop/#!/Combat-Helmets-MK-81LL/p/129037094/category=6272033