There were 8 palantiri, 1 in Tol Eressea, and seven in Middle Earth.
Concerning the seven, (Gondor) the Osgiliath stone fell into the Anduin river during the Kin strife and as of its size, being able to be lifted only by several men, would stay in the exact location where it fell in making it possible to retrieve it.
The Ithil stone was taken by the Ring Wraiths during the capture of Minas Ithil and eventually given to Sauron. When Barad-dur fell, it would have been buried under the rubble of the tower, not crushed as many think, as the only thing that could possibly destroy it was the fires of Mount Doom which are kilometers away, making it possible to retrieve it and use it.
The stone of Minas Anor(Tirith) never left its position and is still there. But it is extremely difficult to use due to the steward Denethor burning himself with it on his pyre. People of great will and power can only use it as anybody else will just see Denethor's burning hands.
The stone of Orthanc is still in perfect condition and is currently in the hands of King Elessar(Aragorn). (Arnor)
The palantiri of Anuminnas(kept at Lake Evindim), and Amon Sul,(kept where the name implies, also known as Weathertop) are lost in the bay of Forochel with the wreck of king Arveudi(last king of Arnor). These stones are not capable by any technology in Middle Earth to obtain them from the sea.
Now the the last one is special because it can only look west to the sea and the undying lands. The palantir of Elostirion was kept in the tower of Emyn Beraid in Ered Luin hidden there by the elves until it was taken across the sea with many noticeable characters including Bilbo, Frodo, Gandalf, Galadriel, and Elrond with many more to the undying lands.