You need to go to the National Archives, Penn. Ave. entrance in Washington, DC, 4th floor, or get a copy of the NASA Militry Service Records: A Seclect Catalog of National Archives Microfilm Publications.
Therein you will find what they hold in their Compiled Military Service Records holdings.
There are many records for Virginia Reserves, Local Defense Units, Militia, etc.
Among those we find: 3d Reserves [Brookers Regiment]; 3d Reserves [Archer's] Battalion; 3d Battalion, Valley Reserves (Augusta County Reserve) The latter is found on Microform M324, Roll 401.