A blanket is a versatile prop for yoga. It provides protection of your neck in shoulder stand and a soft support for your head in forward bends or supine poses. Blankets are the first props used at the beginning of class to lift your spine while sitting for the invocation.
The most popular blankets used in yoga are colorful cotton/rayon blend Mexican blankets or the white cotton blankets from India. To avoid "kabi" or mold spores in the more humid weather of Japan, the rayon and cotton blend Mexican blankets work very well, but I have never had problems with the natural all cotton blankets from India. Some wool blankets also provide a good density for a yoga blanket and are popular in colder countries. A yoga blanket should not be too soft or "fluffy". It needs to compress well and offer stable support.
Indian blankets are available on Amazon but may take some time to arrive from India. https://www.amazon.co.jp/-/en/Iyengar-Yoga-Blanket-India-Shipping/dp/B01IXX06H2/ref=sr_1_41?dchild=1&keywords=Iyengar+yoga+blanket&qid=1629668227&sr=8-41
At Yoga Studio in Kichijoji has Mexican blankets for sale in solid colors= ¥6500ea
and Pattern blankets=¥6000 each
If you have a yoga blanket source you would like to recommend please let me know in the comments and I will add it to this post.