Please check out this great website. Also below is an excerpt from the site.
"Where does Vitamin B12 come from?"
It's very important for you to grasp that in our modern food supply B12 is found MOSTLY in animal products. Well, contrary to what some think, the cow does not actually make the B12, but the bacteria in its gut does.
When a cow chomps on grass, she eats the vitamin B12-enriched soil which allows the bacteria in the cow's gut to make use of the B12 thereby imparting the animal with B12.
And those ancestors of ours who were able to thrive on a plant-based diet without adding any Vegan Vitamin B12 supplements? Well, they were getting their food directly out of the ground still covered in soil -- soil which they were never quite able to completely remove.
Our food today is usually very clean by the time it makes it to our stores, and then we further clean it in our own kitchen (since most of us don't like the taste of dirt). Goodbye Vitamin B12.
If you eat a plant-strong diet, you MUST be sure you're getting the amount of B-12 you need.
Where to get B12:
Fortified foods, many non-dairy foods are enriched with B12