Grape Seeds are planted on the vineyard plots, 4 to a plot, with Farming mastery 12 for plain seeds (14 and 16 for the better grades). The plots open with the Vineyard technology (55 green), and a harvested vine gives seeds back along with its Grapes.
The town Brewery sells them at every tier (15, 20, then 30), the Trademaster at the port has 20, and Herm 12 at his third tier with 10 copper added to the price.