I can’t answer your question directly, other than to say that I’ve seen them on ebay in the past so it might be worth checking there. It might also be worth contacting/visiting the Japanese embassy. They will likely either know of computer stores catering to the Japanese expat community or at least have newspapers/magazines targeted to that community that you can check through for ads.
But, in the hopes of clearing up a potential misunderstanding, I thought I should mention that few Japanese make use of the hiragana on the keys, in my experience. They mostly type in romaji using the normal QWERTY letters. The only real benefit of a Japanese keyboard over a standard English one is that being able to switch between kana/English with a single keypress is mildly more convenient than using ALT+~.
I just checked the Amazon.co.uk site and searched for Japanese keyboard. This was the first entry. It says immediate shipping is available. Are you a member?