Other Palantiri?

[Aiwendil]

It is from The Road Goes Ever On:

No doubt Gildor and his companions (Vol. I., Chap. 3), since they appear to have been going eastward, were Elves living in or near Rivendell returning from the palantír of the Tower Hills. On such visits they were sometimes rewarded by a vision, clear but remote, of Elbereth, as a majestic figure, shining white, standing upon the mountain Oiolosse (S. Uilos). It was then that she was also addressed by the title Fanuilos.

[The Might]

Ok, so back to the topic, there really simply is no proof of their existence, and as such we must believe they did not exist.
The quotes provided in the first post can be easily explained considering the innate powers of the Elves, so there really isn't any reason to think that other palantiri existed in M-e.