Boromir's search for Imladris

[The Mouth of Sauron]

After Faramir and Boromir had their "seek for the sword that was broken" dream, they asked Denethor where Imladris might be found. And despite living in a world where even Barliman Butterbur knew where Mordor was, Denethor was unable to give an exact location for the House of Elrond.

But surely there was an easy way to find out, because at the time of these dreams Saruman, living in Isengard, was still perceived as an ally of Gondor and Rohan - and Saruman was head of the White Council, of which Elrond was a member.

So why didn't Boromir just knock on the doors of Isengard and ask Saruman where Imladris was ?