A lot of people suggest so-called nutritional yeast because that's what they've been told or read, but so-called nutritional yeast isn't something that can be found in an abundance in nature. "Nutritional yeast" is sold in stores, but it's cultured via a manufacturing process.

Better suggestions for sources of B12 would be sources that can be readily found in nature, and that source is animals.

You won't find "nutritional yeast" in nature, readily available for all, as it's sold in stores. Companies manufacture "nutritional yeast" in large portions and then KILL the bacteria (flora), otherwise you'll end up with a yeast overgrowth problem in your body which presents yet another health problem.