Sake kasu is a byproduct of the sake making process. It is the residue left over after the final pressing of the fermented sake. This slightly dried, taffy-textured product is used in a variety of recipes in Japan but most frequently in soups and as a marinade for fish or meats.
Sake kasu may be difficult to find if you don't have a Japanese food store in your area.
You can substitute sake in a recipe but you won't get the same depth of flavor. See this marinade substitute below.If you can find sake kasu, you can substitute this combination for a marinade (from Joy of Cooking):
If you happen to be in California we have a good-size sake production industry and some of the producers sell the lees. Other than that it's going to be difficult to find. If you are in the San Jose, CA region, try one of the grocery stores in JTOWN, one of the three last Japantowns in the United States. If you love the Japanese culture and food, you need to go there, it's a great area!
If you are in San Francisco, drop by Sequoia Sake located at 50 Apparel Way, San Francisco, CA 94124. They will give you Sake Kasu free of charge if you bring your own container during their public tasting hours (12-6 pm on a Saturday). There are sake tastings too for a small fee.