I am from India and we do not get Natto (Japanese dish) starter for making Natto. I found that it contains mostly Bacillus subtilis, so I was wondering: can I mix the Bacillus subtilis strains available in the pharmacy to prepare my starter from scratch?
Bacterial are named by species, not strain. Each species has innumerable strains. For instance, Escherichia coli has over 700 major strains. Some can kill you. Most are friendly.
It is analogous to breeds of dog (over 300). All dogs are Canis familiaris species with numerous "strains" (breeds). Some can kill you. Most are friendly.
So when you pick up a "probiotic" Bacillus subtilis in the drug store, are you buying a poodle or a Rottweiler?
Culinary micro-organisms tend to be very specialized. The lactobacillus found in yogurt is unlikely to be the same lactobacillus strain that is found (along with yeast) in sourdough starter.
I make my own yogurt, beer, malt whiskey and sourdough. I try to source my bugs from commercial food products as close as possible to the taste I am after.
I suggest you make an effort to obtain commercial Bacillus subtilis var. natto and learn a way to propagate it.
Generally this is possible as you can also use probiotic capsules for making yogurt. The important thing is, that it has the same bacteria strains and preferably a similar quantity.
Where you have to take care, though, is that the starter substitute doesn't contain any other ingredients. For example in probiotics are often further ingredients that might cause yogurt to spoil, when using it for this matter.
Look for quality products without any other ingredients