If this is a first time to buy from Amazon Japan refer http://domoarigatojapan.weebly.com/blog/how-to-buy-on-amazoncojp-how-to-register.
for Rakuten Global refer
http://domoarigatojapan.weebly.com/blog/how-to-use-rakuten-global-service
But maybe not needed because it's easy to do.
Here below are items which you can buy through Amazon Japan.