[radagastly]

From "The Council of Elrond:

"So said Denethor. And yet there lie in his hoards many records that few now can read, even of the lore-masters, for their scripts and tongues have become dark to later men."

Granted, Gandalf is referring to lore about the ring, but it implies that even lore-masters (like Denethor) have forgotten many things, including what little they knew of the location of Imladris. It was never well known:

From 'The Tale of Years,' appendix B, "The Return of the King":

1695-Sauron's forces invade Eriador. Gil-galad sends Elrond to Eregion.

1697-Eregion laid waste. Death of Celebrimbor. The gates of Moria are shut. Elrond retreats with remnant of the Noldor and founds the refuge of Imladris.

1699-Sauron overruns Eriador.

1700-Tar-Minastir sends a great navy from Numenor to Lindon. Sauron is defeated.

1701-Sauron is driven out of Eriador. The Westlands have peace for a long while.

c. 1800-From about this time onward the Numenoreans begin to establish dominions on the coasts. Sauron extends his power eastwards. The shadow falls on Numenor.

This shows that Rivendell existed only three years of the war before Sauron was defeated, and that Imladris was established as a "refuge," and Gondor/Arnor were only beginning to be established. Numenor was still at the height of it's power. Across the sea, I suspect maps of Middle-Earth (as much as WE all delight in them,) would have been scarce, if they existed at all. Certainly, none of them would have contained a detailed location of Imladris. It was, after all, a "refuge," in a world where Sauron not only still existed, but could still maintain a pleasing, charming form. It's location, especially since it contained one of the Three Rings, would not have been advertised, especially to Numenor, where "Alatar" resided. Elrond would never have taken such a chance.

So, in Minas Tirith, there would have remained VERY little information to start with, and even those who studied such things (not Denethor, surely. He would have prefered accounts of battles and strategies,) would have had very little information to convey to Boromir before he set out. In fact the only information that he seemed to have, comes from Boromir's own quote during the Council of Elrond:

Of these words we could understand little, and we spoke to our father, Denethor, Lord of Minas Tirith, wise in the lore of Gondor. This only would he say, that Imladris was of old the name among the Elves of a far northern dale, where Elrond and Halfelven dwelt, greatest of lore-masters.

Actually, the fact that Boromir found Rivendell at all may be a case of intervention from Eru. How else could he have stumbled upon such a place, designed to be hidden, concealed by the Elves, an ancient legend, at best, as far as his people were concerned, and probably disguised from outsiders by Elrond's Ring? The odds of Boromir finding it then were about as good as the odds of you finding it now!