War of the Rebellion series ("Official Records")Portal to Texas History: Link to Digitized Images of Source
Volume XXX (Ch. 42, in four parts)
Volume 30 (Ch. XLII, in four parts)
Operations in Kentucky, Southwest Virginia, Tennessee, Mississippi, North America, and North Georgia, August 11-October 19, 1863.
Part 1--Reports (pg. 1-1071) (via Texas History)
Part II--Reports (pg. 1-821) (at Archive.org - Univ. of Va.)
Part III--Union Correspondence, etc. (pg. 1-1017) (via Google)
Part IV--Union and Confederate correspondence, etc. (pg. 1-768) (archive.org - Univ. of Ca.)
Includes "The Chickamauga Campaign"
Reference: Battle of Chickamauga dates: Sept. 19, 1863 and Sept. 20, 1863 (main battle).OFFICIAL RECORDS, SERIES I, VOLUME XXX, PART I - REPORTS.
Official Records, Series 1, Vol. 30, Pt. 1 [Reports].
If not found, also try Cornell University. I've found this site less reliable--it sometimes shunts you off to an auto-generated-text version, which is really bad. Sometimes even the volume numbers are wrong! But here's the link: http://ebooks.library.cornell.edu/m/moawar/waro.html .
PART 1, FULL-TEXT, TEXT FILE ONLY, AUTO-GENERATED, WITH ERRORS
Here is a link to a full-text version, via Cornell University. This version was automatically generated and is full of errors!