You can enter Pet Soul Stone Shop by clicking Merchant in Sanctuary.
Here, you can get Pet Soul Coins by exchanging excessive Soul Stones of pets who have already evolved to the Absolute Star. You can use these Soul Coins to purchase Pet Potions (used to level up pets), Chaos Particles (used to charge pet Stones), and Soul Stones of various heroes. Alternatively, you can use gold to purchase Pet Potions and Chaos Particles.
The list of the goods for purchase refreshes every 24 hours at 9:00 PM your local time zone.
In addition, you can update the list for 50 emeralds. This price gradually increases with each refresh attempt.