The original poster has probably given up or gone home long ago, but I'm pleased to report that I found 100% cranberry juice in Japan! Not exactly in Ikebukuro, though. Where I finally found it was in Matsue, at the supermarket (ordinary grocery store) in the Saty shopping center. This will not be of much help in itself, but its existence there is encouraging. If it is sold in Matsue, of course it must be sold elsewhere (although all I could find in other stores was that nasty 27% sweet stuff, and I looked a lot of places). The pure juice I found is distributed by Marukai as a health food product, so probably health food stores are indeed the best place to find it (although this is probably of more use to people who live in Japan, and not travelers like myself--I never seem to run across health food stores in the places I visit).
The following page has more information on it and a picture of the bottle, although there is nothing in English. The “kodawari inryo” page (second button in the row below the top banner) has a little more information.
It's quite pricey (I paid about 1200 yen for 500 mL). I am not enough of a devotee to spend my valuable vacation time looking for health food stores in every city I visit, but I'm definitely enough of a devotee to spend my money on it if I happen to find it in a supermarket. I consider it to be an insurance policy of sorts.
If anyone else cares at this point, and knows where to buy it in Tokyo (or Okayama, my next major stop), please post. Thanks!