Wheat seeds can be bought at the General Store. Wheat takes 28 days to grow and sells for 100 G each in a Shipment Box. The high sell price makes any villages you come across as valuable, as they often contain chests full of wheat, which can be sold. It must be harvested with a Sickle.
Vanilla wheat will spawn in the world (for example in a village) but cannot be harvested normally. You must buy Wheat seeds and grow it. Wheat cannot be planted the regular way.