For the major metropolitan newspapers of 1920s and 1930s, the best place to begin is ProQuest Historical Newspapers:

Several other important digital collections include the following:

There are hundreds of freely available digitized newspaper collections. These collections vary wildly in quality, but to the historian hunting down evidence, quality isn’t always a top priority: if a collection has the source a historian needs, then he or she will happily use it, especially if it’s freely available online. Many states have their own digital newspaper collections, often developed in tandem with the National Digital Newspaper Program (NDNP). States that have partnered with the NDNP, but that have not developed their own separate collections, are not included here. This list is divided by region: