Asomodai

Any L/XL real helmet should do the job.  If you're after a chinese knock off you're pretty much out of luck. 

See if you can find any one with one of the airframe knock offs you could try on ,my boy has a ridiculously huge melon and it fits him ok .👍

I got the L/XL "Ops Core Tan Sand DE SWAT Tactical Maritime ABS Helmet" from HWAirsoft and it fits me fine, has more padding to stick in it if you need a tighter fit too as well as the dial to tighten it up. I've got all the padding in and near enough tightest setting and I'd have said my head is pretty big so I reckon that'll sort you out!