Scholarly resources may not be apparent if you simple "Google." You can increase the likelihood that an authoritative resource appears at the top of your results by limiting your search by filetype (filetype:pdf) or domain (site:gov or site:edu or site:org). Alternatively, you can begin your search at Google Scholar.
Internet Archive Scholar is a good alternative to Google Scholar. For scientific literature, there is Semantic Scholar.
You might even try Elicit, an Artificial Intelligence (AI) Research Assistant. You'll have to "sign up," but it's free and worth experimenting with this AI tool.
For your Memorials Around DC project, think of entities that manage those monuments or drive visitors to those locations. While the information you need might not appear on the website, consider contacting them for assistance.
Smithsonian Institute
National Archives
U.S. National Park Service
If your group is dealing with a former president, can you find that Presidential Library and ask the staff for assistance?
If your group is dealing with a war memorial, is there a group of veterans that might help you? The U.S. Department of Defense?
Always consider organizations or associations who need to know or want to know about your topic.