Citing to digital legal resources with specificity (and confidence in the URL’s permanence) is tricky business.
Documents and websites have a habit of moving around cyberspace – a lot. Corporations aren’t the only non-human entities that have “people“-rights to move cross-country, so to speak; digital documents change their addresses (URLs) as frequently as human-people do and there is no law stopping them from doing so.
KCLL Klues has this blog post, with references that will give you a good start when researching this subject:
The Wayback Machine and Using Printouts of Archived Web Pages at Trial, January 15th, 2010.
(U.S. Supreme Court: Citizens United vs. Federal Election Commission, 08-205, January 21, 2010. U.S. Supreme Court official website.)