In order to make natto we need to eliminate starter cultures - same goes for making kombucha, yogurt, sauerkraut etc. Which came first the chicken or the egg? At some point in time we need to become self sufficient so that if the time comes where we can't access these said cultures, we can still produce these fermented foods.
The Japanese used Rice stalks. There is nothing inherently special about rice stalks that separates them from other vegetation because all vegetation contains Bacillus subtilis. you could use a corn stalk, grass clippings, dried edible weeds, or even a bale of hay. As long as it's dead and dried out and not poisonous anything is fine; you could use dried out cabbage leaves, etc. Right now I'm using dried banana leaves. (not banana peelings).
The rice stalk or whatever you choose to use is boiled for around 10 minutes. It is very important that this boiling is done at the threshold of boiling. You might be thinking that this would be for sterilization to eliminate foreign invaders from taking over the ferment, but in actuality, it is this temperature that wakes up the Bacillus subtilis. I personally don't care for the idea of boiling because it imbibes the dried out vegetation. I find even after I remove all the water possible from the beans, the dried vegetation will still be saturated from the residual leakage. That is inevitable, so I live more dangerously making my natto just like it was made the very first time it was made by the Japanese. If you're interested in considering this process proceed at your own risk.
In order to obtain a good amount of neba I apply the vegetation that I'm using. I use a pressure cooker to cook the beans and as soon as they are done, I get those beans out as soon as possible. I make sure I drain them completely. I want no standing water; this is all done very quickly so as to keep the beans as hot as possible.
As soon as all water has been removed, a bed of the dried vegetation has already been laid down in a large plastic or glass container (like one you would make lasagna in).
The hot drained beans are poured into this container on top of the dead dried vegetation that entirely covers the bottom of the container. The heat from these beans in contact with this vegetation is what wakes up the Bacillus subtilis. Immediately afterwards another layer of dried dead vegetation is put on top with the lid put on top and almost sealed (I leave it slightly open in one corner). It is now placed inside a huge styrofoam cooler that already has a standard heating pad in it turned on and it is heating up a similar sized container of water. The container of beans rests on the container of water so that the heating pad is not directly in contact with the beans. The lid to the styrofoam container is closed and it will remain around 100°F to 108°F for 24 hours. The closest thing to store bought natto will be made with soybeans or garbanzo beans; although a lot of other beans can produce natto, the flavor will vary. Lentils don't make natto.