1) With Mkvtoolnix, if there is a subtitle in a mkv, you can easily extact it, deselecting all the other tracks and then giving the command “start multiplexing”: it will save that track as a mks file.
2a) If that track is a text one, you can easily convert it in a srt with this command (with a subtitle editor you could have to do another, easy conversion from an ass format to a real srt format):mkvextract tracks "your-mks-file.mks" 0:a-name-you-want.srt.
2b) It the track is an image one (i.g. with one .sup file or two .sub e .idx files) you should use some online tool, such as subtitlestools.com so that from two files, sub e idx, you can one file srt. It could be a long process, waiting for your time aftr other users. But that webpage should keep working even you go offline and the day after you come back to download your srt file.
Sometimes however the easiest way is to find your srt files online, i.g. with OpenSubtitles.