Soul Sand is a hard-to-collect block.
Soul Sand just as normal Sand cannot be mined on any of the public islands, making it difficult to get.
It can only be obtained by unlocking the Soul Sand Trade through reaching Sand Collection III. The Sand article guides through the required steps to progress in the Sand Collection and unlock the Soul Sand Trade.
Once the Soul Sand trade is unlocked, it costs 2 Sand and a single Fermented Spider Eye per piece of Soul Sand. The Spider Eyes and Sugar required for the Fermented Spider Eye can be bought from the Alchemist inside the Potion Shop and Brown Mushrooms can be collected in the Mushroom Desert.