Check the usenet newsgroups there are two groups (I think) where people post Jpop.
A good program to access usenet is FreeAgent. (which is Free).
If you are content to listen to Jpop (without actually posessing it), check out www.live365.com you will find a number of great radio stations playing Jpop 24/7.
There is also great Pop music from Korea (Kpop), China (Cpop or Manopop or Cantopop), Thai (Tpop), don't forget to look around for them too.