B12 is made by bacteria. The only way to get it other than animal products (fortified vegan foods are fortified by animals products) is from poop. http://plenteousveg.com/b12-intestinal-bacteria/
Algae doesn't make B12 in an activated form humans can use. Rats can use it but not humans.
It would seem that the only way to be a non-cheating vegan (fortified B12 foods is cheating) would be to eat poop. But... Would eating feces count as animal products for the purposes of veganism?
Just eat yeast extract.