Actually, you can get a Pufferfish sometimes if you visit the travelling cart that appears down in the woods on Fridays and Sundays. It's near the ranch where you can buy animals and hay. Just hang a left like you're headed for the Wizard's tower but keep to the north border past the big tree.
@David Messner: it's supposedly got more to do in the game besides watering plants. but i can't speak from experience regardless i've only played harvest moon and i lost interest in that about two weeks into it's time limit.
