Where can I buy a copy of the painting by James Walker entitled "Battle of Chickamauga"? My great-grandfather was in the 121st Ohio

You might start by contacting the US Army Heritage and Education Center, Carlisle Barracks, Pa.

121st Ohio Infantry Regiment
“Our Photo Archive includes images of individuals of this uni”

http://cdm16635.contentdm.oclc.org/cdm/singleitem/collection/p16635coll9/id/11307/rec/1