I am not sure as I (like most players) was not actively gathering sprouts.
IIRC, I thing they are given mostly in endless (it used to be gems and coins but sprouts was replacing gems. I think you will also get sprouts from quests.
I knew way back that I occasionally got Marigolds, but I honest to God never noticed the shovel. I was conditioned by old game to accept that plants for garden were hard to come by. And when I got PVZ2, I didn't carefully read the new rules. Under old system you didn't delete plants until you were awarded a unique new plant. You just filled your garden and let all the plants earn coins until you needed the slot.
