Ridgeballs can be obtained only by purchasing the the right Extinct seeds from the supply shop or by mutating a Multiflora. A Multiflora can be obtained by purchasing a specific type of Extinct seed, by mutating a Weeper or Ridgeball, or by crossing a Weeper and a Ridgeball. Those are all Tier 4 plants, which can't be bred from the starting seeds, so you're going to need to purchase seeds or mutate plants to get them. To see what each seed looks like, there's a Seed Catalog in the Guides section.