i just got my first mythi and it says i need a diamondmouth but i dont know where to get it
You can get it most easy in the special ingredient mission that cycles with the crown/moalith/elementary dungeon. You need 50 tickets to open that quest. If you don’t have that yet, try to raise hero rank till then.
Unfortunately you just missed it and you have to wait almost one whole cycle.
Try to get the Emeraldmouth / Amethystmouth in the current Balancion Trials if you still can. It’ll be useful for you later.