Some newspapers maintain online archives of old editions, such as the New York Times and the Los Angeles Times. The Library of Congress also has its own corresponding collections of newspapers issued in previous decades and even previous centuries. For actual physical copies of old Sunday newspapers, antique newspaper dealers or reprints of vintage editions are an option.
The New York Times has a special tool called the Times Machine, at TimesMachine.NYTimes.com, which is a browser-based digital replica of every edition of the New York Times from 1850 to 1980. The online Los Angeles Times store at Store.LATimes.com features vintage reprints and back issues.
To find specific days, dates and editions of newspapers through the Library Congress, click Historic Newspapers at LoC.gov. The website automatically displays the Library of Congress's collections of historic newspapers dating back 100 years to the current date. Adjust the range of years and search within a specific state using drop-down menus, or click a specific newspaper's name under Advanced Search.
Private dealers of old newspapers are another option for tracking down copies of specific newspapers and dates. Stephen A. Goldman and Timothy Hughes are two such dealers. They are located online at HistoricalNews.com and RareNewspapers.com respectively. Both dealers sell through eBay.com, which is another potential online source of old copies of Sunday newspapers.