US National Archives
Nara: Presidential Election Laws: Presidential Election Laws
Check out this site from the National Archives and Records Administration on the US Constitution and the United States Code for laws and provisions regarding presidential elections.
Other
Succession to the Presidency a Chronology
This site is put together by a medical doctor with an interest in the line of succession to the presidency. Although not a constitutional expert, the author has documented source material and has read extensively in the field.
Annenberg Foundation
Annenberg Classroom: Presidential Succession
This website contains an interactive timeline about the history of Presidential succession.
Thomson Reuters
Find Law: u.s. Constitution: Twenty Fifth Amendment
Complete text of the twenty-fifth amendment as its worded in the U.S. Constitution. Annotations can be found at the bottom of the page along with a footnote.