How to download video subtitles ? (for the deaf)

In some videos course there are subtitles, and I would like to know how to download them?
I think it can be very useful for deaf and/or hard of hearing people.

So we can add subtitles on VLC to watch and understand certain video.
Files can be in .cdg *.idx *.srt *.sub *.utf *.ass *.ssa *.aqt *.jss *.psb *.rt *.sami *.smi *.smi *.txt *.smil *.stl *.usf *.dks * .pjs *.mpl2 * .mks * .vtt *.tt *.ttml * .dfxp or .scc  .