The Japanese recipe for fermented soybean, called Natto is an abundant natural source. Not easy to find in the US though. I've found it offline at Japanese groceries and one boutique grocer at an exorbitant price so far.
I had heard that myself but was a bit disappointed in the very mild flavor and coffee-like smell. Needs a bit of soy and mustard to give flavor. Not as odd as people said imho, and more accessible than kimchi. It is gooey however.
Possibly good for prostate health as well. Another reason to try I forgot to mention.