I want to provide this information since most of the cosmetics I am writing reviews on, can be purchased only in Japan.
Here is some useful guide how to shop on Amazon Japan.
This is how to shop on Global Rakuten.
You can also buy using shopping services such as: Rinkya, Celga, Shoppingmalljapan, Tenso, Japangoodsfinder.
If you have any experience buying goods from Japan, please share it below in the comments. I would highly appreciate that and add it to this post.