The original subtitles were burned-in to the picture, so it’s unlikely that a file of just the subtitles is out there.
…not that we can’t try. The latest Project Threepio includes graphical subtitle files derived from 35mm scans. Not perfect, but not far off the mark.
Ooh! I stand corrected. I knew Harmy ripped 4k subtitles off of RotJ, but I didn’t know you had done work yourself. I suppose I’m a bit behind on a few projects.