This library database from ProQuest contains historical documents pertaining to the African-American experience from 1790--the 2000's. Around 2,000 primary source documents can be found and accessed. It's searchable, but you might find it easier to choose collections from the pre-designated time periods, many of which already coincide with the tabs listed here. As this is a paid library database, you will need to login with your MU login credentials to access.
This database focuses on the writings of major figures of African-American hsitory: politicians, teachers, war veterans, religious leaders, and more. These writings include book exerpts, letters, speeches, etc. Another paid library database, you will need to log in with your Marian credentials to access. This database is searchable but also browsable by subjects, events, titles, people, places, and even content type.
Over 100,000 maps and related images from every country, some dating back as far as the 1500s. Just type your keyword(s) into the search box and see what comes up. From battle maps to railroads, if it can be mapped, it just might be found here. Use the Who/What/Where/When limiters on the left after you search to help refine to your search.
Though it may look as if you need to log in, you can click the orange Explore Primary Source Documents to search for your topic. You can refine by selecting a pre-set era, narrow by type, and sort by relevance, title, date, or popularity.
Indiana Memory is provided by the Indiana State Library, and archives state sources including newspapers, memoirs, and even works by local political cartoonists. Searching is easy; the box for typing a search is hard to miss. While difficult to separate out different formats, the results are easy to see what they are, with thumbnail previews. Broad topics can be narrowed down to more specific ones on the right.
The Library of Congress houses digital materials covering a wide range of topics in American history, many of which are primary sources. Clicking the link below will take you to a searchable page; just type your topic into the search box in the top right corner. You can limit your results by source type such as Newspapers, Film & Video, and even Personal Narratives. Keep an eye on the source dates, though, as not everything will necessarily be of the era you want (though a memory project that records the stories of people who have lived through a time period or event would still be primary source material, even though the records may be from a much later era).
Library of Congress Digital Collections
Newspapers are an excellent source of primary information. This digital collection of scanned newspapers spans from 1756-1963 and can be searched. Individual thumbnail previews can be clicked to enlarge.
Library of Congress Chronicling America: Historical American Newspapers
The United States National Archives Catalog is a digital repository of multiple types of primary sources. Easily searched (the search box on the home page is impossible to miss!), you can scroll to the bottom of the limiter list on the left side to Dates by Year to help narrow your search results to post 1860 dates, in 10 year increments, as needed. Want to see a wider range? When you click on the 10 year date, it provides a To date and a From date; you can bypass the 10 year increment by deleting the To date. Try it out yourself with Westward Expansion by clicking the 1860-1869, then deleting the To 1869.