Is there a reliable, automatic subtitle downloader for Mac OS X?
I know this is an old question, but I was searching for this same answer, and found a great solution. 

Subliminal will allow you to set up a cron job, (or manually enter from CLI) whole directory trees, and download subtitles. You can also set it up as a post processor for SABNzbd or SickBeard or CouchPotato.

It's working great for me.