The best dietary source of B12 is, surprisingly, clams.
Beef liver is another good source, though i would only eat this if organic. Salmon & other fish, beef, dairy & eggs are also good sources. Many grains & cereals are fortified with b12. The most reliable vegan source is nutritional yeast
but i advise vegans to take a B12 supp. See http://ods.Od.Nih.Gov/factsheets/vitaminb12-healthprofessional/.