If you buy one, the EmersonGear fast helms have a good rep for the most simplistic level of helmets, but they have two types, basic - interior is one lump of polystyrene(i think) padding, and no adjustment screw, and advanced, which has removable pads and a size screw, measure your head before buying!
Generally they're a rip off in this country anyway, so something better might be affordable, but don't ask me what, I started my airsoft hobby copying popular youtubers - I do not regret it.
Note - as you will see from other forum topics (see search) these helmets are replicas and should not be treated as an object expected to protect your head in a situation you'd expect them to, that said, tree branches door/window frames, a basic knock into a wall, and BB's they're great. Just don't go jumping off a mountain in them - probably shouldn't ride a bike either.