There are two types of farmer vendors, one of them have all seeds. They normally stay nearby one another, though. I usually go to Brittany Estate to buy seeds. The first building on the left.
The farming merchant at the docks in Central Brittany has them all. Code: Cost Water Each* Yield Vendor Carrot 8 4h 8 2 Garlic 8 4h 8 2 Lettuce 8 4h 8 2 Pepper 8 4h 8 2 Potato 8 4h 8 2 Pumpkin 6 4h 4 3 Tomato 8 4h 8 2 Roses 10 4h 7 1 Banana 16 8h 16 2 Corn 16 8h 16 2 Lemon 16 8h 16 2 Mandrake 16 8h 16 2 Nightshade 16 8h 16 2 Onion 16 8h 16 2 Wheat 16 8h 16 2 Grapes 16 8h 16 2 Cotton 16 8h 9 4 Apple 30 12h 27 2 Coconuts 30 12h 18 3 Tea 30 12h 18 3 * Inside a greenhouse Edit: Sept 18 2017 Updated 30 gold seeds they're watering windows are now at twelve hour intervals inside a greenhouse.