A note from the Kumamoto International Desk of Cuisine (located near but not in the kitchen as my wife doesn’t allow me entrance unless unwashed dishes await, in which case I’d better respond with alacrity): Japanese sour cream is both expensive and too stiff for most western recipes. She found that, blended at a 2.5:1 ratio (or so - experiment to your taste) with plain yogurt, which is much cheaper, the resulting consistency and taste is remarkably like American sour cream.
That is all. - William