The Titan Soul Shop becomes available once you have evolved at least one of your titans to the Absolute Star (i.e., to the sixth 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 1 Titan/Supertitan Soul Stone = 10 Titan Soul Coins. Once you open the shop, the game will offer you to trade the excessive Soul Stones.