To be honest, you're going to find a lot more goodies by digging through unregistered stuff rather than the drops - at least, that's usually the case although of course just like in the non-IDN market some good stuff drops.
Start from e.g.
http://www.yahoo.co.jp/ and browse the directory. All the URLs (below the first-level ones) are in English even if the categories and subcategories are in Japanese, so a combination of mouseovers and looking at the address bar should help keep you roughly straight.
http://dir.yahoo.co.jp/Business_and_..._and_Services/ takes you straight to all the ecommerce sites - you can get a ton of keywords by poaching off their navigation menus.
Be on the lookout for "サイトマップ" at the bottom of sites - it means "site map" and will often give you a hierarchical, all-text overview of the site. Again, lots of potential keywords.
Make yourself a nice long list, convert it to punycode, run it through a bulk whois checker to see what's still available, then check the availables on Overture Japan's bid tool, keyword research tool, Google Japan (put the entire expression between "" marks) and in the dictionary (links below).
http://uv.bidtool.overture.com/d/sea...idtool/?mkt=jp
http://inventory.overture.com/d/sear...estion/?mkt=jp
http://www.google.co.jp/
http://www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi
Oh, if the word/phrase isn't in the dictionary but it shows up with a big Overture score and the top site exactly matches it, then it's likely to be a trademark/brand rather than a generic.
That should get you started - takes a bit of getting used to, but once you hit your stride you should be able to churn out 10-30 candidates an hour.