Unless you are vegan, a B12 deficiency is caused by another problem. B12 is easily present in all omnivore diets whether it is a SAD, a perfect 100% harm-free diet, or anywhere in between, and a store of B12 after B12 is not eaten anymore can last for up to 20 years. If you have been vegan for less then 5 years, or if you are not vegan, then you have a health problem which is causing the B12 deficiency to be possible, such as leaky gut (which, in itself, can have another cause, such as low stomach acid causing the leaky gut).
If you are vegan, you need to very frequently eat nutritional yeast or legitimate probiotic products (such as real, alive kombucha, yogurt that you have made yourself, or sauerkraut that you have made yourself). These are the only sources of B12 that exist that are vegan.