If a source is not listed, it can generally be assumed to be one of these:
A New Nation Votes: A compendium of election data for a variety of United States offices; anything between the years of 1787 and 1824 (as well as House elections through the end of the 19th Congress) can generally be assumed to have come from here if another source is not given.
The Clerk of the House of Representative's Election Statistics: A collection of the official vote counts for federal elections from 1920 to the present. Other information from the House Historian (such as party divisions) will also be used, as it is official and authoritative.
Presidential Ballots, 1836-1892: A collection of county returns for president from 1836 to 1892. Unfortunately, the third-party vote is only broken down in 1860 and 1892, and even then only one third party is distinguished.
Dave Leip's U.S. Elections Atlas: A compendium of various statewide elections, largely for President, Senate, and Governor, for which post-1990 coverage is fully available. Do note, however, that this will more be used as a base, and if better information is found it will be used instead.
The entire bibliography of Michael J. Dubin: Dubin, the man who inspired me, was the authority on county-level returns, with presidential elections up to 1860 and gubernatorial elections up to 1952. He also has a book on Congressional elections by district up to the end of 1997, and of state legislative party divisions up to 2006. Do note, however, that his works will more be used as a base, and if better information is found it will be used instead.
The Green Papers: A site on American electoral practices with an entire section dedicated to the history of the process. Election dates will primarily be gleaned from here and/or Dubin.
Wikipedia: They have historical Congressional rosters. It would honestly be somewhat foolish to exclude them.
A Century of Lawmaking: A collection of old Congressional records, to be used for balloting for legislative offices (mostly the Speaker of the House).
The Documentary History of the First Federal Elections, 1788-1790: A collection of documents documenting the 1788-89 elections for President, the Senate, and the House.
The Alabama Legislature: The legislature provides legislative journals up to 1828 in HTML format, as well as all six of Alabama's constitutions.
Party Politics in Alabama from 1850 to 1860: This book provides gubernatorial and congressional returns from 1847 through 1859, as well as legislative rosters and party divisions for the same period.
The Alabama Department of Archives and History: The Archives provide legislative maps from statehood up to the 1960s.
The Alabama Department of Archives and History's digital collections: The Archives also provide the following:
The Secretary of State: The Secretary of State provides election data from as far back as 1946, though full coverage only begins at 1986.