If you haven't yet found a health-food shop that carries what you want, your best bet is probably going to be one of the many vitamin and supplement suppliers on the Internet. I've found one that sells pretty much the same brands you get in the US, but it seems to be limited to Europe.

Do an online search for one or more of the brands you like, and add "Japan" to the search criteria to see if you can pick up someone who ships (regularly) to Japan. Or you could order from the US directly, though if they aren't used to shipping overseas, the shipping charges will probably be outrageous.
Cheers,
Bev