You probably mean the one showing Thomas' defense of Snodgrass Hill, which is used on the cover of the current NPS folder for Chickamauga-Chattanooga NMP. According to the credit on the folder the painting is "Courtesy National Museum of the U. S. Army, Army Art Collection." But I wouldn't be surprised if the NPS store at Chickamauga had copies for sale as prints. As I noted in my separate thread about the painting itself, there is also a much smaller version of Lookout by Walker on display in the Fuller Gun Collection room of the museum/visitor center there.
Yeah that is it. I believe David Powell is also using it on the cover for his new book about Chickamauga. I'd love to get a print of that, may just have to look next time i'm up at the battlefield in the Visitor's Center.