From the Vatican Web site, links to the encyclicals (and biographies, letters, etc.) of John Paul I, John Paul II, John XXIII, and Paul VI.

Citation Example:

MLA

John Paul II.  “Encyclical Letter VERITATIS SPLENDOR addressed by the Supreme Pontiff Pope John Paul II to all the bishops of the Catholic Church regarding certain fundamental questions of the church’s moral teaching.”  6 August 1993.  Papal Documents  Hekman Library, Calvin College. 19 Dec 2002 <http://www.vatican.va/holy_father/ johnpaulii/encyclicals/documents/>.