I will Beavis. :g The only web site for legal download that I know of is Stingray Digital. Sound Choice sold much of there past music library rights to them for downloads. The unfortunate thing is they only offer it in MP4 or WMV with the lyrics or an audio only MP3.
Chartbuster has had their download sales page up for a long time, but all it says is it is coming.
Buykaraokedownloads may look legit but do not have anything to back it up. They have Terms and Conditions like everyone has but that does not say they have rights/permission from Sound Choice, Chartbuster or anyone else. That would make me suspicious of their legality.