The book I just read on fermented foods has instructions for making miso (which looks considerably more involved than I think I have patience for; it has to age a year before it's ready to even taste), and one of the things the author mentions is that you need to buy the proper inoculants (koji fungus, and I think something else too) to inculate your batches with, but that the home or shop of someone who regularly makes miso will have sufficient spores for it just hanging around in the air that it can be left to wild-ferment.