Innolife sells Korean music. One of the comm I'm member of recs YA or DVD heaven, which has LOTS of k-music, and another comm recs CDjapan and brandx.jp (in japanese) but I've never bought from them.

Oh and there's also Amazon Japan ^^ iTunes carries some j-pop, not much though. Again, I can only vouch for YA, itunes and amazon. Oh oh oh and HMV has an online Japanese site which sells lots of Japanese things and they ship to Europe. I've bought stuff from the UK version of that site and they are reliable. Again, can't vouch for the Japanese version, but hey, I suppose if the service if good from one it follows that it should also be good from the other.