Mochiko (もち粉) is a type of glutinous rice flour (also called sweet rice flour) that is made from mochigome (もち米/糯米, glutinous short grain Japanese rice), and it’s sometimes called Gyuhiko (求肥粉) or Daifukuko (大福粉). To make mochiko, mochigome is first washed, drained, then ground into very fine flour, and dried.
There are rice flours using long grain rice from other countries, but it tastes different and texture is very different from mochiko so for me, they don’t work well to make Japanese sweets (wagashi).
If you can find Shiratamako (白玉粉) shown above, which is another type of glutinous rice flour, I highly recommend to use it instead of mochiko because it’s much easier to use and texture/flavor is nicer. If you’d like to know the difference between these two rice flours, please hop over to Shiratamako page to read more details.
You can buy mochiko on Amazon, Japanese supermarkets, and some Asian stores.