This sounds too weird to be true. I mean, can't you just imagine it, and then search for it?
The answer is mostly not.
Recently there was the tale of the long-lost letter of Descartes showing up in the Haverford College library. It's a heartwarming story of a letter that was stolen in the 19th century from a European library, then making its way to a small Pennsylvanian liberal arts college, to rest quietly in their library.
Last year, a Dutch scholar, knowing of its existence, decided to do a Google search for the letter. Lo & behold, search returned a result for a catalog entry in the Haverford Macgill library.
It's a good story... but it makes the point: If you don't think to search for something, you're never going to find it!
Alas, we're hemmed in by our cognitive mind sets more than we're limited by the capabilities of search. As the Descartes letter incident shows, just because a document isn't online doesn't mean you can't find it.
For instance, one of my favorite books is Dream Machines: New freedoms through computer screens--a minority report. (Self-published book by Theodor H. Nelson, 1974) It isn't easily available, although you can buy a used copy on Amazon for $95. And many libraries just don't have a copy.
But it's pretty simple to find via Google Books. Just do a search on Google Books, then click on the "Find in a library."
And I quickly discover that it's at Stanford, just a couple of miles away. (Google can do this by handing off the book information to WorldCat, which is a meta-catalog of many libraries around the world.)
I've written about "Find in a library" before, but this time I want to emphasize that if you don't think to look, you'll never find. Don't let your imagination limit what you might try to search for.
My current favorite: [ why is a raven like a writing desk ]
(I like it because it's a riddle from Alice in Wonderland, and the story written on this by Cecil Adams of The Straight Dope, is wonderful.)