It's not necessary to make a starter when making sake, provided you're following the traditional method of making a moto or shubo (both are yeast starters) a couple weeks before starting the moromi build up.
Depending on where you live, it may also not be necessary to make your own koji from spores. Koji is used to make other things than sake (like miso, amazake, and soy sauce), so any store that supplies groceries for this kind of Asian cooking may possibly stock prepared koji. Cold Mountain sells their koji in 20 oz tubs, two of which will supply exactly the 2.5 pounds of koji you will need to turn 10 pounds of rice into ~3 gallons of sake.