Your desire is shared by many. However...
1. Unless the music is in public domain, it would be illegal in most countries for someone to place Karaoke songs on an Internet page for downloading.
2. A typical Karaoke CD+G song is 30MB to 60MB in size. A full disc would be in the range of 650MB-750MB. With the fastest Internet connection, it would take hours to download. This is not reasonable with today's technology.
As Brian suggests, you might try our Karaoke Home Producer software that allows you to make your own Karaoke songs. It can be a lot of fun making them, and then again... singing them.
