How to cite a reference which disappeared?
I personally think this is not your responsibility to control the sources availability over time. If the official provider's URL is added, it's usually enough. There are numerous means to retrieve the metadata; the easiest one, as mentioned by @JeffE, is WbM, also there is Google cache.

BibLaTex supports a urldate = {YYYY-MM-DD} field, which adds "visited on MM/DD/YYYY" info to the reference in bibliography, assuring the reader which date can be used to restore access if the link is dead.

More technical aspects regarding handling external data sources with BibLaTeX are discussed on TeX.SE: URL of cited web site in bibliography