Depends on the language your are looking for I suppose.
But the first place I always go to (for english) is subscene.com. Make sure you are looking under film title and not release name.
I always look for 'official' rips - either bluray rips or dvd rips. anything else - like xvid or whatever seems to never match up for me.
One thing to keep in mind that has been driving me nuts here lately is extended versions. it seems like most groups like ripping the extended versions and not the theatrical. So now when i rip a br that has both, i double check what subtitles are listed first before ripping.
if you are looking for chinese - shooter.cn is a great site for american movies. Sometimes they include both chinese and english srt files which is handy.