Thanks for the OP. I purchased successfully from Bookwalker on Android tablet. It seems to have to sufficiently large purchase choices. (Kindle JP failed for me - 😫 Kindle frustrations! Registering an amazon.co.jp account - #40 by polv)
Actually, I have known another source for a while. ヤンジャン！ Android app.
- I can’t install this one from my Play Store. I have to use an APK, which is a pain when the app updates. (Newer APKs eventually appears, but it lags behind a little, by a few hours, actually.)
- I can get some of purchasable contents for free, via in-app Blue Coins, which I can gain by watch advertisements (which appear to be some broken ads in my native language).
- I can somehow pay (by buying gold coins) without creating an account at all. (As I have the app on both smartphone and tablet, this is a problem, as it does not sync.)