Great question? In food the best sources are going to be beef, fish, shellfish, milk, and eggs. Some sources will say that cereals are a good source of Vitamin B12, but know this, cereals are fortified with B12 and other nutrients and may be synthetic or not so bio-available versions of B12. In eggs, most of the B12 is in the yolk. For fish, the fatty fish like salmon, mackerel, sardines, trout will have the most. But other fish have a significant amount also. Beef organ meets like liver have a substantial amount of B12. But if you don't like liver, a good ole steak will have more than enough for you.
From plant sources...it's really hard to get B12. A lot of vegans will get it from fortified nutritional yeast and cereals. The following is a quote directly from the Vegan Society web page,
"Very low B12 intakes can cause anemia and nervous system damage. The only reliable vegan sources of B12 are foods fortified with B12 (including some plant milks, some soy products and some breakfast cereals) and B12 supplements, such as our very own VEG 1. Vitamin B12, whether in supplements, fortified foods, or animal products, comes from micro-organisms. Most vegans consume enough B12 to avoid anemia and nervous system damage, but many do not get enough to minimize potential risk of heart disease or pregnancy complications."
If you eat a fair amount of animal products especially the ones I mentioned above. From Healthline, "
One grilled flat iron steak (about 190 grams) provides 467% of the DV for vitamin B12."
If you take a drug like Metformin or any kind of stomach acid blockers you might want to get checked out by the doc and get your B vitamins checked at your next visit. If they tell you to take a supplement, most B Complex vitamins will do. But I am picky about what I take. I pretty much limit most of my supplement usage to two brands, Now and Jarrow. Currently I am taking this B6, B12, and Folate supplement:
https://www.amazon.com/Jarrow-Formu...words=jarrow+b12+and+b6&qid=1650073666&sr=8-5
If you do take a B12 supplement, try to get one that has the Methylcobalamin. It's a very bio-available form of B12.
I am also linking a B12 fact sheet from the NIH that I think is pretty good.
And the last two links at the bottom are from the quotes earlier in my post. I hope this helps a good bit. It at least gets you started. Let me know if you have any questions!
Vitamin B12 is an essential nutrient your body can’t make, so you must get it from your diet or supplements. Here are 12 foods that are high in vitamin B12.
www.healthline.com
The critical importance of vitamin B12.
www.vegansociety.com