Adventurer Helmet may well be one of the rarest items in Remnant: From The Ashes. In fact, it seems to be so rare that it might not even exist at all.
If you're wondering how to get the Adventurer Helmet in Remnant, we think we're going to have to disappoint you, as there's been no confirmed report of anyone ever getting it. At least, none that we could find ourselves.
Remnant: From The Ashes Adventurer Helmet
Chances are that you've already checked out our full Remnant armor set guide beforehand, where we state that it seems to be impossible to get the Adventurer Helmet/Hat at all. We're sad to say that, since that guide, we've come across no information to suggest otherwise.
Someone reported a while ago that the Adventurer Helmet dropped as a random purple drop on Earth, but they've offered no proof of that actually taking place.
We think that, as it stands, the Adventurer head piece simply cannot be found by any means, which may be either intentional or accidental. Either way, it certainly seems like it won't be part of any common combat loadout in Remnant anytime soon. Perhaps the Adventure Mode update brings this issue up, but also maybe not. We'll have to wait and see.