How did Celeborn, Glorfindel and Beregond get their information ?

[Inziladun]

Originally Posted by The Mouth of Sauron

As a prelude to the Company leaving Lorien, Celeborn seemed to know that "the bridges at Osgiliath are down and the landings held by the enemy".

The destruction of the bridge and the events in Osgiliath were relayed to the Council of Elrond by Boromir, who had witnessed them. After the Council, it is said that Elladan and Elrohir went 'down the Silverlode into a strange country'. They would have told Celeborn what was discussed at the Council.

Originally Posted by The Mouth of Sauron

When Glorfindel found Aragorn and the 4 hobbits prior to reaching Rivendell, he stated that news had reached him from Gildor's folk.

My guess would be that Gildor sent one or two of his company to Rivendell. It was nearly a month from the time Frodo left Bag End til the hobbits and Strider were found by Glorfindel, so there would have been ample time.

Originally Posted by The Mouth of Sauron

And during his initial conversation with Pippin, Beregond started that things "were moving" beyond the Inland Sea and north to Mirkwood and beyond.

Denethor was regularly viewing the Palantír of Minas Anor. Though he kept secret the source of the intelligence he gathered with it, I think it likely he discussed the things he learned with his advisors. Bits of that information could have filtered down to the populace in Minas Tirith, if they weren't outright told what was going on.