The mandate of the Supreme Court "The judicial power shall extend to all cases, in law and equity, arising under this Constitution" is stated in Article III, section 2 of the Constitution. Decisions and opinions rendered by the Supreme Court can be found in:
FindLaw: In addition to opinions of the Supreme Court from 1893 to the present, it includes biographies of the justices, Supreme Court history, recent court news and more.
Federal laws are codified in the United States Statutes at Large. The printed edition is avaiable on the refernce shelves of the library. In addition v. 65 to v. 126 can be accessed online.