Corruption dust is a crafting-material that can only be obtained from corrupted creatures; which currently means Corrupted Pigsies, Corrupted Chizzards, Corrupted Leafies, Corrupted Mirus and Things. Nearly every corrupted creature will drop corruption dust in it's loot-bag when being killed or dismissed, and you can also get corruption dust very often as a pet-harvest from tamed corrupted Pets. Of course you can find corrupted animals in the Corruption layer, but they will also spawn on corrupted blocks of all kinds whereever you place enough of them together.