what you need to do is either:
1. mutate something you do have into something that you can use to make that plant with, or
2. buy a seed from the store that you can use to make the plant with.

to figure out what you need to make something, it helps to understand how breeding works. the child plant you get when you breed two parent plants can be found halfway along a line drawn between the two parent plants on the chart.

This Post does a pretty good job explaining how that works. (If you find that the halfway point on your line falls between two plants, pick the plant that is more to the right and the bottom.)

Cheers,
-Borg