Titan Soul Shop becomes available once you evolve at least one of your titans to the Absolute star. You can find the store in the bottom-right corner of the Guild Island area:
Here you can spend Titan Soul Coins to purchase Sparks of Power, gold, and Titan Potions.
You get Titan Soul Coins by exchanging Soul Stones of absolute evolution titans. The exchange rate is 10 Titan Soul Coins for one Titan/Supertitan Soul Stone. Once you open the shop, you will be offered to trade the excessive Soul Stones.