If you havent bought your tickets already I would just purchase a Multi City ticket, going onto S. Korea from Japan, so you have something saying you will be leaving Japan. Keep in mind though you will have the same issue with S. Korea, I think officially you always need an onward ticket, whether the customs people ask for it or not is another story. I don't know where you are leaving from but I just bought my tickets from LAX to Tokyo then on to Kuala Lumpur for like $900 on Korean Air. If you arent really sure how long you want to stay in japan I guess you could always change your ticket but that can be pricey. From looking at airfare from US to Japan and then onto some other city in Asia I found that Korean and JAL were the cheapest. I used
http://farecast.live.com/ to find my airfare, might be worth a look.