As a practical source of B12
s today, fortified foods or supplements are probably the only answer. However, the quantity needed has been drastically overstated in many places. As Trevor Goodchild
says above, only a very small amount is required by our bodies. However, this amount is not so small that we can blithely ignore it and count on increasingly unlikely happenstance to give us enough B12.
If you are vegan and pregnant, definitely make sure you're getting enough B12. Pregnancy places increased demands on your body for nutrition, and vitamin deficiencies during pregnancy have terrifying possible effects on your child. In the case of a B12 deficiency, rare though it is, the result may be a type of anaemia - treatable, but why subject yourself or your child to it? It is very possible to stay vegan while pregnant and breastfeeding, but care is required. Consult a doctor or nutritionist who has experience with vegan nutrition, and talk to vegans you know who have been pregnant while vegan.
If you are a lifelong or almost lifelong vegan, or if you have been vegan for more than 10 years, definitely make sure you're getting enough B12. If you ever ate meat for more than 10 years in your life, your body will have enough B12 to last you up to another 20 years or so after you cut out animal products, but don't wait too long before getting in to the habit of getting B12 in your diet.
But the good news: it's really really really easy to get B12. Just eat fortified nutritional yeast. Red Star brand is widely available in health stores, and contains B12 cultivated from vegan sources. Spirulina powder is another good source, although the way it turns anything you add it to bright green can be offputting. It's good in smoothies, and a teaspoon more than meets the RDA for B12. You can also try vitamin supplements, but be careful: many vitamin capsules contain gelatin, and even among those with vegan-friendly capsules, the B12 may have been produced with non-vegan methods. It all depends on how strictly vegan you are, but do some research. If you ask me, though, just skip the supplements and eat nutritional yeast. It's delicious, it has 1001 uses in vegan cooking, and it only takes a little bit to satisfy your B12 needs.