Can I echo the suggestion about midi files? There are still millions there - try a midi search engine like the one at vanbasco.com.
It's really not difficult to adapt midis for singing to - use a sequencer, or a dedicated karaoke player like vanbasco's (above) to mute tracks, transpose etc. And you can convert them to Wav files to burn on a cd by recording them in real time using any old sound recorder - set your recording options to "mixed output" to record directly off your soundcard.
If the midi synthesizer that comes with your soundcard is a bit ropey, then you could use direct music for playback - I believe Winamp has a directX plug-in, and there are other good softsynths available, either free or going for a song (!)
Good singing