Shiratamako (白玉粉)is sweet or glutinous rice flour, or mochiko, mixed with a little corn starch or potato starch. If you can't find shiramako, you can use mochiko with about 1 tablespoon of cornstarch or potato starch flour added. You can find all of these flours at a Japanese grocery store. You might be able to find them at a health food store too, since rice flour is more popular nowadays as a gluten-free thickening agent for sauces.