ArchiveGrid is a database of "nearly a million" collection descriptions contributed by thousands of repositories across the United States—and it's available to the public for free through the end of May. Locked up in libraries, museums, and archives are "historical documents, personal papers, and family histories" few people ever see. Plug the surnames you're researching into ArchiveGrid, and these hidden treasures will be hidden no more. Use the search tips in the left sidebar for best results; I found the proximity search especially helpful when searching for surnames within a given state (e.g. "Dunham Maine" ~10).