Hello! Original author here. I'm not sure about the best tool for creating new .srt files, but for films, you can find pretty much anything on opensubtitles.org or subscene.com (although the quality varies). You can also download .srts for youtube videos (again, quality varies).
I will caution people that downloaded subs are very often misaligned (timing) with your video, due to various cuts of videos, intro logos, disc versions, framerates, etc., etc. Leave your download page open and check one-by-one against the source video.
