A few weeks ago I was passing the only Japanese food store I know of in the Toronto area, and I forgot to pick up some ready to serve Natto up so as to try Eva's method. I don't doubt it would work, especially if blendered after treating the blender with beer brewing type sanitisers like Idophor or Five Star Starsan.
However after a year of sporadically searching, I was able to find a source in the US willing to ship the actual concentrated culture to Canada.
http://naturalimport.com
As far as the "Natto Fail" thread which I just read, with the 3 year old Gem Cultures packet I had on hand, I used a picnic cooler with elevated hot water containers and a digital readout cooking probe to maintain the approximate 100 degrees F called for. After more than 48 hours the result was not optimal, but I still think beneficial and more or less edible. If you drown it in enough tamari, dijon mustard, and green onions, it tastes sort of OK.
BTW the author of the book I referred to was saying people were using black beans instead of soya successfully.
As far as if Natto is beneficial, I think this link to a fairly long conference video will convince people:
Ken