If you're interested in finding a Japanese girlfriend or wife then there aren't quite so many options as there are for the mail order bride type Asian countries (by these I mean Thailand, the Philippines, and to some extent, China). One popular Japanese dating site is Japan Cupid. Although there are plenty of Japanese ladies on this site, Japanese ladies tend to be fairly shy, and not many ladies like the idea of posting their profiles on a site like Japan Cupid.
Another possibility is to use a more introductions based approach. One alternative to Japan Cupid is to use the Transpacific Marriage Agency (TMA). This Japan based company helps Western men look for Japanese wives.
The Transpacific Marriage Agency works more like a dating site rather than a introductions agency. Men and women register, and the women are sent the men profiles, and men are sent the women's profiles. Some of the men and women are also listed on the TMA website.
Be rest assured that Transpacific Marriage Agency is a legitimate business, and plenty of men and women have found love using the service. I've seen some of the ladies on other dating services, so I know they're genuine.
I was interested in finding a Japanese wife, so I joined TMA for a few months. To be honest I'm not sure if it's worth the money really. There are some good ladies on the site. However, the site works by sending old fashioned PDF files of profiles to the members. These have low quality photos, and it's hard to judge a woman by a small thumbnail in a PDF file. I also found other shortcomings, namely that it wasn't always clear where a lady lived. Traveling around Japan is expensive, and it's often better to stick to finding a lady in an area you know. If your Japanese isn't great then venturing outside of central Tokyo, Osaka or Kyoto is pretty difficult as well.
So what's the alternative to the Transpacific Marriage Agency? One good tip I picked up from them is that in the membership pack they give all kinds of information away about how to find Japanese ladies. Sadly you'll really need to be living or working in Japan to use these tips. But in reality if you want a Japanese wife then you'll sort of guess that it's much better to live in Japan anyway. There aren't many Japanese women who are interested in living overseas. Personally I would say that Chinese ladies are a better proposition. Of course Chinese and Japanese cultures are vastly different. But if you're after a beautiful, smart and sophisticated Asian lady, then you might just be stunned at the quality of ladies on Chinese Cupid.
Update: I left TMA as it was largely a waste of money. Quite a few of the women are on Japan Cupid anyway. In fact, Japanese dating was a big waste of time for me. Why bother with Japanese women when there are tens of millions of hot Chinese women desperate to chat to you online?