I take sublingual B12 supplements when I go mountain climbing -- mostly just for a quick energy boost at high altitude.
But, yes, eat real food -- and, I love this site:
Top 10 Foods Highest in Vitamin B12 (Cobalamin)
GREAT list of Top 10s!
Top 10 for B12:
1: Clams, Oysters, Mussles
6: Crab and Lobster
My understanding has been that B12 is animal-exclusive.
I'm actually working on a B12 deficiency -- dangerously low levels.
I was curious so I've been pounding around a bit after reading your post and I find this:
Link to First Site:
Mushrooms cultivated on manure enriched compost will contain vitamin B12. If the mushrooms are not over washed before use they will contain some B12. (Yeah. Sounds to me like this is because they still have "animal shiitake" on them...)
There is 0.26ug of vitamin B12 in 100g of mushrooms. A serving of 4-6 mushrooms weighs 75g.
Link to Second Site:
Assuming that the B12 is an active analogue, it would take anywhere from 7 to 326 cups of mushrooms to meet the RDA. (Sounds like a shiitake-load of mushrooms! And nobody likes a recycled joke...)
Link to Third Site:
If they are Inactive Analogues they are worse than useless, possibly harmful.
Link to Fourth Site:
Inactive B12 analogues in plant foods compromise the accuracy of traditional methods used to determine the vitamin B12 amounts and activity of a food. Bacterial contamination of a food can create the false impression that all such foods contain B12 analogues. The only reliable way to determine if a food is a source of active B12 is to test various batches of that food to see if it reduces methyl malonic acid (MMA) levels in humans. (It appears that no such study has been done...)
Personally, I love me a good siitake but I'm guessing you might wanna bail them -- at least as a plan for a source of B12.
Are you vegetarian/vegan? If not, start pounding the clams!