Gentry County doesn't appear to have a genealogical or historical society nor a museum. Sometimes these organizations have historical info about businesses.
You can contact the Gentry County Library, www.gentrycountylibrary.org/G.C.L
You might find an add for it in a City/County Directory.
Another possible source is their local newspaper. Missouri newspapers that are online can be found at the Library of congress, http://chroniclingamerica.loc.gov/newspapers/missouri/
If you don't find it online, you can contact your local public library and ask about inter-library loan of newspaper microfilm. Successful Searching!