Dear Ms. O’Kelley,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located a series titled Records of Appointments of Postmasters, 1832-1931 and Records of Appointment of Postmasters and the Establishment of Post Offices, 7/1/1931 - 10/31/1971 in the Records of the Post Office Department (Record Group 28) that includes the file Postmaster Appointments for Sebastian County, Arkansas which has been digitized and is available online through the Catalog.
These records also are available through Ancestry through their website as data collection U.S., Appointments of U. S. Postmasters, 1832-1971. Some libraries provide free access to Ancestry to their patrons. The copies on Ancestry are name searchable.
For more recent information, please use the USPS Historian’s Postmaster Finder.
We hope this information is helpful. Best of luck with your research!