Here’s an unpleasant thought: just half a gram of dried human fæces contains the daily requirement of vitamin B12. The bacteria that produce vitamin B12 inhabit fæces in particular, and as a result, the rabbit eats its own fæces, the second stomach of the cow eats the fæces of the first, and humans, traditionally grew their crops with ‘night soil’.
Perhaps for similar reasons, other sources include duckweed; algæ, including chlorella and coccolithophores; seaweed, including nori, aka laver, and dulse; along with animal products.
The consumption of animal products is a practice intertwined with human history, culture, and health. However, it is worth knowing the origin of these products, particularly in the modern era where the production techniques tend to come as a surprise, and raise questions of whether these products can be considered healthy, despite the presence of various nutrients.
Even when avoiding meat from modern sources, there are still associated health risks, particularly regarding red meat and the colon.
Besides that, the land requirement is steep. For example, if everyone in Britain obtained their B12 from home-raised poultry, while otherwise following a basic diet, then there would be land for around 45 million people, with even less for beef, lamb, pork, and cheese. With a population of 65 million people, meat would not be a viable source of B12 for Britain as a whole, not when self-sufficiency is considered and factory-farms are avoided.
Seaweed is a good source, but a lot is needed. Otherwise, algæ and duckweed are widely available, replicate rapidly, and have impressive nutritional profiles. However, they are rarely used in food. Instead, it may be useful to look at them as ways of making natural supplements. For synthetic supplements, note that more than a few micrograms may be harmful.