Dungeon Shop unlocks at team level 30. You can find it by tapping the Shop button in the bottom-right corner of the Dungeon screen:
You can also select the Dungeon Shop tab when you open Merchant on the main game screen:
Here you can spend Dungeon Coins to purchase Titan Skin Stones, Fairy Dust, Titan Potions, and Soul Stones of Corvus, Alvanor, and Isaac.
All goods are updated every 24 hours at 9:00 PM your in-game time. You can also refresh the Dungeon Shop using emeralds.