That's one of the species that cannot be bred from other species; it must come from seeds purchased from the supply shop or by mutating another plant. If you do decide to mutate (and hope for the best, since the results are somewhat random), you would have the best chance of success if you start with another species that's somewhat close to the Citrus Scandens, such as a Venomous, Mela, or even Fourpetal Scandens (or Grass, Orchid or Astera). The "farther away" your starting plant is from the Citrus Scandens, the less likely it is that you'll get the right mutations (and you may have to use a lot more Mutation Liquid to get what you want).