It's a career reward in the biology career track. You can't buy it. There's one at the secret society at Sim State campus.
Specifically: career level 5 (Soil Identifier) of the Natural Scientist career. If you are doing a custom university, it is available on all 3 Secret Society lots.
Just because I don't know if you play the original game or the EP: there are collections out there with buyable asp-rewards but for the cow plant to show up in your game you need University -le
