I just used a prefab rice koji since I’ve only done that once.
https://www.amazon.com/MIYAKO-Malted-making-Pickles-Isesou/dp/B004FH67ZQ/ref=asc_df_B004FH67ZQ/?tag=hyprod-20&linkCode=df0&hvadid=309811627177&hvpos=&hvnetw=g&hvrand=16641520446120917709&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9061082&hvtargid=pla-392633965407&psc=1