well, lets see. There is yesasia (they have chinese and korean stuff too) jpophouse (but I have heard they are not very good) amazon (sometimes has stuff buts its would be realllly expensive I would use it as a last resort) CDjapan (one of the best)
^ Myself and no one I've heard of has ever had any problems with jpophouse.com. The CD singles are too expensive though IMO. I like how there's "Free shipping for orders over 10$) JpopHELP.com is supposedly horrid though.