Sashiko thread is made of cotton, is relatively thick, and has a matte finish. Unlike embroidery floss, it is spun so that the threads should not be separated; it is used as is.
If you want thinner thread, you can buy it here, but this thickness is the "traditional" one. The thread is from Olympus Japan, a company over 100 years old! In other words, they have a lot of experience :)
Sashiko thread is a bit more "fuzzy" than embroidery floss, and adheres nicely to the fabric you are sewing, blending in over time as an integrated part of the material.
Instructions for sashiko will eventually be available on this website, but it will take some time. Many have wanted sashiko supplies for a long time, so I'm choosing to sell them before I can provide instructions in Norwegian for those who want them.
Search for sashiko on YouTube, and you'll find some good help! There are also dedicated sashiko groups on Facebook, for example, and several handicraft guilds offer sashiko courses, so check a handicraft guild near you if you're interested! The book "Mending Matters" also covers a lot of sewing inspired by Japanese handicraft techniques.
You can find needles and thimbles for sashiko here :)