Are the great mirrors and lenses of historic telescopes, especially those of Herschel, Rosse and many others back from the 18th and 17th centuries, preserved? The wooden structures of those telescopes, I assume, rotted and became firewood, but did they preserve the mirrors and lenses?
Many of these objects are collected in museums. The German "Deutsches Museum" for instance keeps famous original instruments of Fraunhofer http://www.deutsches-museum.de/sammlungen/meisterwerke/meisterwerke-ii/refraktor/ Such institutions are scattered all over the globe.
Further there are many museums devoted to individual astronomers like https://en.wikipedia.org/wiki/Herschel_Museum_of_Astronomy keeping original relics and replica.
Most relics will have been preserved at their original places. The Göttingen Sternwarte keeps original instruments used by Gauss. A list of such institutions would get fairly long.
One of the most important mirrors historically is that of Sir Isaac Newton's telescope shown below. In the Periodic Video Amazing piece of metal (speculum), Professor Martyn Poliakoff (CBE, CChem, FRS, FRSC, FIChemE) describes the telescope, notes about building the telescope, and what is believed to be the speculum mirror used in the telescope, at the Royal Society in London.