Titan Soul Shop becomes available once you evolve at least one of your titans to the Absolute Star. You can access it by tapping Merchant on the Guild's main screen:
Here, you can spend Titan Soul Coins to purchase Sparks of Power, gold, and Titan Potions. You can also exchange Titan Potions for a Golden Horn of Summoning here:
This shop cannot be refreshed, there is no update time, and goods are always available and the same. So you can buy as many items as you want.
You get Titan Soul Coins by exchanging Soul Stones of titans with Absolute Star. The exchange rate is 1 Titan/Supertitan Soul Stone = 10 Titan Soul Coins. Once you open the shop, you will be offered to trade the excessive Soul Stones.