Looking for a way to download closed captioning files for a deaf family member.

If you have a title such as a film that is in mass media a subtitle source may have one for your language.  Then the player can pick up the subtitle and add it in during play.

A search for 'add subtitle'  etc. will turn up the usual subtitle sites with lists
across many languages.

I'm not aware of specific instances of CC files from third party sources other than the usual SRT file extensions.

Easiest way I have found to add cc.
Get VLC video player.
Play the video
Go to view
Go to VLsub
Download file
Works great for me