The U.S. Constitution is the most authoritative primary source. All statutes, regulations, court opinions, etc. must be in accordance with the U.S. Constitution.
Locating the U.S. Constitution
The text of the U.S. Constitution is reprinted in the United States Code (U.S.C.). For more info on the U.S.C., see the Statutes page.
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Each state has its own state constitution. You can usually find a state's constitution within its statutory code (generally reproduced immediately before the initial title).
Cornell's Legal Information Institute has links to state constitutions.
In Pennsylvania, a copy of the State Constitution can be found right before Title 1 of the Consolidated Statutes. It is, of course, available on Westlaw or Lexis, as well as for FREE from the PA General Assembly website.