As a specific example, From playing a Druid in DCI Adventurers League games (specifically "The Quest for Sporedome") I have a magic Item cert for a set of Half-Plate of Poison Resistance (Highest AC medium armor). The armor is explicitly usable by Druids, as it specifies on the cert that the plate is made from petrified giant mushrooms.
And seeing as these adventure modules are now being sold, officially, via WotC's Dungeon Master DriveThruRPG site, they are definitively published official first party supplements.
So, you can make armor equivalent to metal armors out of non-metal materials, they are simply rare and require either finding in odd places or deliberate questing to find materials to make them.