There are 2 version of the level 50 class gear; I believe that nowadays, when you complete your level 50 class quest they give both your final "normal" chest piece as well as the full "advanced" set.

You can buy any of the "normal" pieces from a Calamity Salvager NPC, so that will give you something to put on your head for now: https://ffxiv.gamerescape.com/wiki/Gallant_Coronet

To get a replacement for the "advanced" version you were wearing, you'll need to get the dungeon drops as mentioned by Kirika above. If you want to farm that's a fine choice and might be a fun adventure! But if you'd prefer to get it back fast I imagine it would be pretty easy to find a bored level 90 player who would be overjoyed to have the opportunity to help you get it back by annihilating the dungeon unsynced with you.