The Internet has damn near everything, ever. Between what you have to pay for and what you can find for free, pretty much every song ever recorded, every movie and TV show ever filmed and every book ever written is available somewhere. But what about the stuff that is mysteriously absent?
For me, that would have to be a clean sample of the red alert from the alt-Enterprise D in Yesterday's Enterprise. Of all the vast terabytes of arcane Trek-related stuff out there, for some strange reason this doesn't exist.
I mean, some clever guy went back and actually recreated SFX from Space: 1999, but this seemingly obvious Trek nugget is apparently unavailable anywhere.
What item have you searched high and low for but never found online?