Two top sites for legally purchasing karaoke music you can use are sound choice and total karaoke. I’m not sure if you want to get the rights to music so you can make it your own and individualize it, or if you’re just looking to use the music as it is, but you can purchase tons of karaoke music in bulk through these sites.

Here’s a karaoke anti-piracy website. You should probably have a look so you know what not to do.

This link tells you how to request rights for karaoke songs.
Here’s another link that might tell you something.

Hopefully the above is some kind of help. Good luck to you through your venture! :)