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 it adheres nicely to the fabric you are sewing into, blending in over time as an integrated part of the material.
Instructions for sashiko will be available on this website eventually, but it will take some time. Many have wanted sashiko supplies for a long time, so I choose to sell them before I can provide instructions in Norwegian for those who wish.
Search for sashiko on YouTube, and you will find a lot of good help! There are also dedicated sashiko groups on Facebook, for example, and several craft guilds offer sashiko courses, so check a craft guild near you if you are interested! The book "Mending Matters" also covers some sewing inspired by Japanese handicraft techniques.
You can find needles and thimbles for sashiko here :)