Seeds can be won from playing mini-games. Most games work on a 10 points to 1 seed ratio.
Most items need 30 seeds to plant. However, some of the lighter and smaller objects might need less. The very tiny buttons only need 4 seeds. Light things like Candy Wrappers only need 15 seeds.
Some rarer items might cost more. The guitar picks need 50 seeds, and the passive soldier needs a whopping 100 seeds to plant.