Soul Stones Shop becomes available once you evolve at least one of your heroes to the Absolute Star. You can access it by tapping Merchant on the main game screen:
Here you can spend Soul Coins to purchase Artifact Chest Keys, Summoning Spheres, Huge EXP Potions, hero equipment, and Jet's Soul Stones.
Note
Soul Stones Shop is the only place where you can get Jet.
New goods appear every 24 hours at 9:00 PM your in-game time. You can also refresh the Shop using emeralds.