Perma.cc is a free to use service where you can create a link to the version of a web page from which you obtained information and ensure URLs listed in your citations do not lead to broken links and missing pages.
“Perma.cc, developed by the Harvard Library Innovation Lab, is a caching solution to be used by authors and journal editors in order to integrate the preservation of cited material with the act of citation. Upon direction from a paper author or editor, Perma will retrieve and save the contents of a webpage, and return a permanent link. When the work is published, the author can include that permanent citation in addition to a citation to the original URL, or just the permanent link, ensuring that even if the original is no longer available because the site goes down or changes, the cache is preserved and available.”
– ‘Perma: Scoping and Addressing the Problem of Link and Reference Rot in Legal Citations’ by Jonathan Zittrain, Kendra Albert & Lawrence Lessig (Harvard Law Reform Forum, March 17th 2014)