There are far more mp3 files being shared than wma, lots of people won't download wma
What you should know about wma files

You could try searching for artist - wma or artist - .wma & see of that brings up some results (try album & song tiltle too, although searching for song title is liable to bring up more spam if you're looking for newer releases).
Make sure that wma has not been added to your keyword filter (tools > options > filters > keywords).