Supplements if you're vegan, there are vegetarian sources.

Just looked up the Wikipedia page to check on vegan, and… looks like all the animal and plant "sources" actually ingest the stuff from elsewhere?

  Since only bacteria and some archea possess the genes and enzymes necessary to synthesize vitamin B12, plant and algae sources all obtain the vitamin secondarily from symbiosis with various species of bacteria,[5] or in the case of fermented plant foods, from bacterial fermentation.[86]

TBH fortified yeast flakes are a pretty good spice.