The China town in SF has about one or two stores there that sells J-music CD's. Surprisingly. You could try your search there, if you'd like. When I was walking by the stores, I heard either Ayu, or Ai Otsuka playing in one of the stores.
But I also second Kinokuniya as well. It's your best bet. Even though the prices may be a bit high, it's your best try. Albums range around 30-40 dollars. New albums, such as Ayu's GUILTY and Koda's KINGDOM, are a bit higher considering they are really famous artists and it's new.