I really don't know, since they are still a new crowns plant. But try the Wayvern's Hoard packs, I got a Key Lime from it. And those two plants came out the same time.
I'll look it up right now! According to the wiki, it is not dropped by any bosses, but it might be worth putting a little money into, as apparenly at Elder harvest it will always give 1 Couch Potato seed, and pet snacks! So you can always have a Couch Potato, and pet snacks!
