You can buy karaoke discs all over the web. Just go to a search engine and tyre in "karaoke discs". Sometimes you can find a good deal on a set of disc from Ebay.
DOWNLOADING KARAOKE: Keep in mind, if you are not paying at least $1 per song (new) then it is very very illegal and immoral. There are no downloads that are legal in the USA, yet. Now that MP3's are legal to buy in the USA over the web, I feel confident that karaoke tracks will follow, but not right now.
There is a ton of great advice from very seasoned karaoke hosts on the MTU board. Be sure to take the time to read their advice,
Have Fun!
Jim