A few months ago my local Costco actually had Banryu Honjozo to my surprise, and I could find it at one of the Whole Foods supermarkets, so not a bad idea to check WF as well if JKB lives near one, but if he’s out of state then that’s a different story.
Another approach is if you find a decent bottle of Junmai, Ginjo, or Honjozo, drink some to taste and/or to pair with food, and cannot or don’t want to drink the rest, leave it in the refrigerator and use the leftovers for cooking. You could in theory use it in e.g. Taiwanese recipes that call for “rice wine” (e.g. for sesame oil chicken or what not) and just swap it with sake.