You can find kogin thread in a wide range of colours here!
The threads are primarily intended for sewing kogin stitches, which you can read more about here.
Since they are made of cotton and can also be split like embroidery floss / embroidery thread (unlike sashiko thread which must be used as is), these can also be used to mend and repair clothes made of cotton or other materials where you want a matte, neutral thread and a wider range of colors than cotton darning thread.
The fact that the threads can be split means you can adjust the thickness to the fabric you are repairing.
Do you need fabric by the meter for kogin? You can find it here.