If you would like to check to see if your ancestor was in the 8th Missouri Volunteer Infantry (US), here are some websites with Civil War rosters:

THE CIVIL WAR SOLDIERS AND SAILORS SYSTEM (maintained by the National Park Service) is a computerized database containing very basic facts about servicemen who served on both sides during the Civil War.   The initial focus of the CWSS is the Names Index Project, a project to enter names and other basic information from 6.3 million soldier records in the National Archives.   The facts about the soldiers were entered from records that are indexed to many millions of other documents about Union and Confederate Civil War soldiers maintained by the National Archives and Records Administration.

The DATABASE OF ILLINOIS CIVIL WAR VETERANS OF MISSOURI UNITS (maintained by the Illinois State Archives) indexes names of 5,610 Illinois citizens appearing on the rosters of Missouri's Federal Civil War units.   Illinoisans occasionally joined military units raised in other states.   In December 1863 the Missouri Adjutant General compiled rosters of Illinois men, extracting the names from his unit rolls.   He then sent them to the Illinois Adjutant General.   The rosters are a part of the Archives’ record series, Company Muster Rolls (RS 301.019).
 
Fred Delap of Kansas, Illinois, created and donated this database to the Illinois State Archives.   Delap is a volunteer with the Edgar County Genealogical Library.

REMINDER:   Besides the 8th Missouri Infantry (US), there was also an 8th Missouri Infantry (CSA).