Vitamin B12: Very good Information sources (by Office of Dietary Supplements of National Institutes of Health)

http://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/

http://ods.od.nih.gov/factsheets/VitaminB12-QuickFacts/

Vitamin B12 is found naturally in a wide variety of animal foods and is added to some fortified foods. Plant foods have no vitamin B12 unless they are fortified. 

by Office of Dietary Supplements of National Institutes of Health