This should give you free japanese books offered through kobo:
http://www.kobobooks.com/search/sear...ne&g=both&l=ja
You can also get newspapers in japanese, by using calibre and going into Fetch News. Go to the Japanese section and pick when to download, or just download a copy immediately.
Note the issues with mainichi and the fix in this thread...other japanese newspapers seemed to work fine.
Edit: the kobo books may not be anything that interests you, but since you said you wanted it for testing, I figured I'd supply that source anyway. There may be someone out there who knows a free japanese books source that is good....so chime in if you're that someone!