I agree that KJ Pro is the best catalogue out there but it has a limit.
MusicBookDeluxe is another that is almost as good, it has some that KJ pro missed, but in general KJ gets more of them
http://www.musicbookdeluxe.com/
The main limit is that both only have the songs produced as Cds, but a few of the minor manus in the UK and elsewhere only sell their songs as single songs downloads or custom disks. While some of these small manufactures make cds their production runs are very small.
Due to US laws few of the makers of karaoke in the USA can produce the very small runs of songs that they can sell of obscure songs.
The major UK manus (Zoom and Sunfly) are generally in the above databases.
Some of the notable makers of Karaoke that are not on the above databases is SBI karaoke, and Music factory both have a bunch of unique songs.
http://www.sbikaraoke.com
unfortunately very expensive (I spent 65$ on a 14 song cd)
Music Factory
Tracks can be found at tricerasoft:
http://www.tricerasoft.com/karaoke_songs/songs.html
Beyond that it gets very hard to find the oddball songs.
A few bands have put a karaoke song or two on some of their cds/dvds, especially their Asian releases as special extras. They tend to be very hard to track down even when they were major releases.