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)

http://samples.mplayerhq.hu/karaoke/

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.

The manual can be found here:
https://usa.yamaha.com/files/download/other_assets/5/328515/cvp609_en_om_c0.pdf

If a particular file contains lyrics as well, they can be displayed using the "Lyrics" function (Page 61).

As far as actual song downloads (I do not know if they contain lyrics),
This site has a fair amount of songs:
https://www.markheadrick.com/midi.php

I have previously downloaded the massive archive listed on this reddit page:
https://www.reddit.com/r/WeAreTheMusicMakers/comments/3ajwe4/the_largest_midi_collection_on_the_internet/

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.

The manual states you can also use an iPad or Mac to connect to the Clavinova, plus the downloads page has drivers and software for Windows.
https://usa.yamaha.com/products/musical_instruments/pianos/clavinova/cvp-605/downloads.html#product-tabs