Since I don't do well with Japanese, all the site I found were commercial producers or sake breweries that weren't offer their compounds.
I check Gem Cultures and saw nothign about koji at all. Since they work more with tempeh I would not think they would be a good source.
However...... check out this link.
KOJI to Brew Sake
If that ends up being a dead end, maybe more searching for homebrewing of sake will turn up more sources for koji, rather than looking for it to use with food products.
BTW, you just inspired a culinary experiment. I am going to attempt a soup that is a cross between hummus and miso. We'll see how it goes.