There are very few legal sources for CD+G downloads. I would also think it would be against this sites plocies for anybody to contribute to any type of illegal activities. Back some years ago I wanted to take songs off different CD+G disc and combine them into what would be our Favorites discs. That way we would not have to keep changing discs each time we wanted to sing another song. In my search I did find what I was looking for. I can only assume these links are still useful as I've abandoned all discs and went to a PC based system solely. The PC systems are so much easier and you don't have to worry about losing or scratching your favorite disc ever again.
Try this link for download of songs. Each song will cost a $1 or $2. This site also has some tutorials on how to make use of the downloaded files.
http://www.cavsusa.com/cinet_home/#
Try this link site is the one I used to make my compilation CD+G discs from years ago. As mentioned I can only assume the info there now is still good with todays softwares.
http://www.karaokeplayground.co.uk/help/help.htm
Good luck to you and I hope I didn't break any forum rules by posting these sites. If you want to go to a PC based karaoke system I'll be glad to steer you in the right direction. All the softwares are free and any other costs would be associated with the hardware. But for less than $20 you can convert what you already have and a home stereo system into a good PC based set up. Let me know if I can help any.