The original, most authoritative source on these in pre-digital times was the Oxford English Dictionary.  And they assembled all of their references manually, although they can be said to have invented crowdsourcing, as people from all over the world.contributed.

The OED is still an amazing source.  Today, though, companies like Google and efforts like Project Gutenberg are making great efforts to digitize everything we have, and then it's only a 'grep' command away.  Like Tuffy said, a good deal of everything we ever wrote is lost, though, and having studied ancient Greek myself, it's tragic.  We've lost countless works of some of authors we, and even everyone at the time, knew were masterpieces.  Try searching for Sappho, so many fragments.  Whole plays by Euripides, gone.