You can only craft the mask at the Skyforge in Whiterun. I did this in order to avoid cluttering every forge in the game and also because I felt that as the Skyforge is the most powerful forge in all of Skyrim, it would make sense that a powerful item like Mask Of Corvo could only be forged there.
You need the Arcane Blacksmith perk in order to forge the mask. The mask should be found under "Steel" in the forge menu.
You can also upgrade the armor rating of the mask at any workbench thereafter.
•Glow Dust can be found on Wisps/Wispmothers and can be purchased from relevant vendors or stolen. •Fire Salts can be found on Flame Atronachs and can be purchased from relevant vendors or stolen. •Small Dwarven Metal Plates can be found on Animunculi or in any of the numerous Dwemer Ruins.