Look for free, but legal sources of karaoke music online. Some "free" music is little more than low-quality MIDI files with lyrics attached that has often been created illegally. MIDI files also require good-quality sound cards to make those files sound their best.
Download high-quality MP3 files from reputable sources such as Karaoke.com.
Purchase karaoke music CDs from reputable online sources with good selections of the kinds of music you enjoy.
Buy custom-made karaoke CDs of hard-to-find tunes or special combinations of tracks from sites like the MyCDG website.