First, you have to browse allsubtitles, opensubtitles and whatever -some of these sites are full of ads and are even ugly-coded enough to kill your fragile webbrowser. And afterwards, you may find that, in fact, the subtitle you have relates to another version of the video you are attempting to watch, and this results in delay issues.
Not to mention that you may even use a video player dumb enough to force you to always select the subtitle (.srt, .sub, etc) each time by hand while its name is consistent with the video file name.
There is, finally, a work around. It is called SMPlayer. It is a frontend to MPlayer that have a menu that allows to automatically download subtitles or even set, on the fly, delay for the current subtitle file used.
On a decent GNU/Linux system, you could probably do apt-get install smplayer. For Microsoft Windows, go to MPlayer HQ, select download MPlayer with SMPlayer GUI. Mac OS X users… well… should take spend some time to port it – it shouldn’t be so hard.