Cite and Source is a term for the process that journal staff use to check the citations in articles to be published for substantive accuracy, technical accuracy, validity and grammatical or typographical errors.
The Bluebook requires citation to print sources "unless there is a digital copy of the source available that is authenticated, official, or an exact copy of the printed source" (this often means a PDF, though sometimes an authenticated or official source can be a website if the governing authority has designated it as such). If you know where to look for the print (or PDF) version of the most common types of sources, you can locate these quickly before moving on to sources that are not as common.
If you can’t locate a cited document, don’t hesitate to ask the library reference staff for assistance. If possible, bring the complete article text and footnotes when you come to the library. Context often helps to identify a source, and text and footnotes not assigned to you may be helpful for your footnotes. The reference librarians can suggest search strategies, help decipher abbreviations (or you could try Bieber's Dictionary of Legal Abbreviations or World Dictionary of Legal Abbreviations), and help find difficult to locate sources.