Elephind is a good starting point for searching through freely available archived newspapers. You may search across all newspapers, or narrow by country or collection. If you narrow by collection, be sure to include Chronicling America and the Missouri Digital Heritage collections.
Microfilm holdings in Ellis Library (1879-)
Print Index in Ellis Reference (1987-2007)
If you have a borrower card for St. Louis County Library, you may access Proquest's online archive of the St. Louis Post-Dispatch, or you may visit a library in that system to use it.
Microfilm holdings in State Historical Society of Missouri (Sept. 1885-)
Current newspaper subscribers may access the online archive (1880-1950) through the KC Star's archive portal.
Suggested strategy: look for events associated with your person or her organization in a smaller newspaper or book; then look at microfilmed newspaper from that time.
Cardholders at Daniel Boone Regional Library may log in to Newsbank to find articles from the Boone County Journal/Ashland Journal, Fulton Sun, Kansas City Star and St. Louis Post-Dispatch, but only for about the last 20-35 years of each paper (no early 20th century).