I haven’t worked with any WMA recently but . I don’t have any on my computer to test with but I bet dbPowerAmp , f2k , or Audacity can all do this . If set correctly all will produce a good quality .mp3 . Of course the .wma file has to be good quality to get a good .mp3.
Edit : I didn’t check first to see how old this thread was . If I had I probably wouldn’t have posted.