Koji, is the fungus, aspergillus oryzae, that grows on grain substrate. It is used in many East Asian cuisines to ferment food. The rich enzymes of koji break down starches and proteins of grains into sugars and amino acids, for making various miso, soy sauces, sake, amazake, etc.
The koji starter I use here for making koji rice is Light Rice Koji Starter from Gem Cultures in US. If you live in Canada, Culture Mother is going to launch those koji starters soon. In Japan, the koji starter can be purchased online, like Amazon.jp, Kawashimaya, 丸福. In Taiwan, you can find various koji starters at Da Shan and 錦池。