I know what you mean, I use both Napster and iTunes and both are terrible for world music. I personally like J-pop, Cantopop, Mandopop and Algerian Rai music and many of the songs I like are not available.
If you don't mind buying CDs, yesasia.com are pretty good for most types of asian music. I have purchased several Ayumi Hamasaki CDs, a concert DVD and a Morning Musume DVD from them and they have always been pretty good with prices and delivery times. Amazon also have plenty of world music at good prices but again, you have the inconvenience of having to wait for them to be delivered.
If you don't mind the less than legal methods of getting the music, 4shared can be a good site or if you have the right software you can simply download a music video from youtube and rip the audio off and save it as an mp3, though the audio quality will vary depending on the software used and the quality of the video.