There are four key materials to sashiko - needle, thread, thimble and fabric. As you'll see, no embroidery hoop is necessary, which makes sashiko a very portable craft.
Traditional sashiko needles are very long (about 2 inches) with a small eye. The width of the needle is uniform all the way to the top. Modern sashiko needles have a larger eye so the needle can be more easily threaded. In the photo, I included the largest embroidery needle in my collection, a 1mm width needle to show the difference between the needles. As you'll see below, a long needle is necessary to the sashiko embroidery technique.
Sashiko thread is more twisted than embroidery floss and not made to be separated into strands. Sashiko thread doesn't have a sheen as embroidery floss or the Valdani embroidery thread have. Either thread could be used a substitute for sashiko thread but the look will be slightly different.
This is a traditional sashiko thimble that is used to push the needle through the cloth. We have a similar thimble made of leather which you can find here.
The ideal fabric for sashiko embroidery is one that is not too tightly woven, such as Robert Kaufman's Essex fabric, which is a linen/cotton blend. Because sashiko thread is so thick, a fabric that is too tightly woven will show puckering or the holes quite easily.