Use Bluebook Rule 12 for statute citation rules.
It is important to note what the author is citing- statutes have a "life cycle" in terms of their printing and an author might be looking at the versions before they were placed in the US Code.
Slip Law: statute is assigned a Public Law Number (Pub. L. No.)
Session Law: Book of all laws (the "Statutes at Large") printed in one legislative session; law as passed before codification so can sometimes provide more context
U.S. Code: The U.S. Code is contains federal laws currently in force, and the Code is published every six years. The most recent edition of the United States Code is 2018, but supplements are released annually.
Using the title number and section, locate PDFs of the current U.S. Code from one of the following databases. If appropriate for context, pull any related Supplements as well.