Dear A. Sharp,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located 2 items titled Milligan, Lambden - State: [Blank] - Year: [Blank] in the Records of the War Department Collection of Confederate Records (Record Group 109); and Charges and Specifications Preferred Against William A. Bowles, Andrew Humphreys, Horace Heffren, Lambdinn P. Milligan, and Stephen Horsey in the Records of the Office of the Judge Advocate General (Army) (Record Group 153) pertaining to Lambdin P. Milligan and his co-conspirators.
We suggest that you review these websites: Ex parte Milligan: 71 US 2 (1866): Justia US Supreme Court Center; IHB: Lambdin P. Milligan and Dissent in the Civil War; Ex parte Milligan; and Review of the Decision of the US Supreme Court, in the cases of Lambdin P. Milligan and others, the Indiana Conspirators for additional information about the Supreme Court case and Lambdin P. Milligan.
We hope this is helpful. Best of luck with your research!