The War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies (usually referred to as the Official Records) were originally published by the Government Printing Office (1880-1901) in 127 volumes plus a general index and an accompanying atlas. There have been several editions. Very few owners of the entire 128-volume set break up the collection, which makes it hard to find specific campaign volumes at a reasonable price.
The Savas Beatie reprints, beginning with The Gettysburg Campaign (vol. 27, parts 1-3), reproduce campaigns of particular interest in facsimile editions of the original series.