The Ridgeball can be obtained only by purchasing the right seeds or by some lucky mutations. Depending on what plant you start with, you can get that foliage by mutation, but it's usually very expensive to do. If you don't already have Tier 4 plants, it will take a lot of mutation liquid and luck to get all the way from a Ball Cactus to a Ridgeball. You might be better off watching for some Tier 4 seeds in the Supplies Shop to work with.

You can see some more specific paths to a Ridgeball here.