Still online research, but, for whatever it's worth...
From the Online Etymology Dictionary...
However, according to the Wordsworth Dictionary of Phrase and Fable...silver bullet
"very effective, almost magical remedy," 1808. The belief in the magical power of silver weapons to conquer foes goes back at least to ancient Greece (e.g. Delphic Oracle's advice to Philip of Macedon)....which tends to indicate that the oracle was talking about a bribe, not an actual silver weapon."With silver weapons you may conquer the world." The Delphic oracle to Philip of Macedon, when he went to consult it. Philip, acting on this advice, sat down before a fortress which his staff pronounced to be impregnable. “You shall see,” said the king, “how an ass laden with gold will find an entrance.”
In fact, A Thesaurus of Traditional English Metaphors translates "With silver weapons you may conquer the world" to mean "Money overcomes all".
There are numerous claims that the idea of silver weapons being superior against any foe (and against supernatural ones) goes back to ancient times, but I haven't been able to pin down any specific references or dates other than the oracle's message (which sounds like it's probably a mistaken reference).
I also found some claims that silver (on it's own) was considered a sacred metal by some cultures, that it has magic(k)al properties, that it was associated with various gods going back to ancient Greece and Egypt, that it had some signicance in alchemy, and that it was associated with the moon.
And that's about all the time I have for research today. Got to get some work done.