I used this a couple of times when I needed some characters for my book.
http://www.springhole.net/writing_roleplaying_randomators/japanesenames.htm
You are probably better off just using actual names and just mixing them a bit to get something unique, randomizing them with this, or at least checking that the name does indeed exist on some webiste. I mean, I doubt people would actually care much about it if the names were a bit strange, (half of the people reading whatever this is might not know Japanese, and won't notice if the name is odd in the first place,) but if you want accuracy and what not, then its better to be safe than sorry.