Which model Clavinova do you have? [EDIT: just saw the model number CVP 605 in the post title, I'll check out the manual]
I thought I had a a link to a large collection of karaoke files, but I am currently unable to find it. I only found this small collection, which may or may not be compatible with your model (most are in cdg format/not midi, but there are a couple of .midi/.mid/.kar files)
So it looks like it supports pretty much every music format (supports, GM, Roland GS, and Yamaha XG) so if you just need regular files with no lyrics, you can download pretty much any midi file and save it to a USB memory stick and load it in the Clavinova.
This site has a search that will let you find files and specify that they contain lyrics: http://www.vanbasco.com/midisearch.html
It links to files on other sites, so results may vary, but it will bring you to a large collection of other sites that have downloads. My midi player doesn't seem to be showing lyrics for a lot of the songs, but you can try some and see what you get.