Dragon Quest II
In Dragon Quest II the Helmet of Erdrick is obtained by trading the Token of Erdrick to am old man in the monolith south-east of Tantegel. The helmet has a defense boost of 20 for it's wearer.
Dragon Quest IX
Erdrick's Helmet may be obtained through alchemy, by alchemising 1 Rusty Helm, 9 Glass Frit and 1 Lump of Orichalchum