Catalogs, Online Indexes, and Databases usually allow you to search by subject, in addition to keyword, author, title, etc.
When you find a good source, look for subject headings in its record to find more related sources. Library of Congress Subject Headings are a "controlled vocabulary" and are used by many resources, including the Vassar Library Catalog, Worldcat, and HathiTrust.
African Americans -- Kansas -- History
Grassland birds -- subheadings
Great Plains -- subheadings
Indians of North America -- Kansas
Indians Of North America -- Great Plains - History
Kickapoo Tribe of Indians of the Kickapoo Reservation in Kansas
Railroads -- United States -- subheadings