I would contact the Indiana State Archives. In the past I have done research on a GAR post there. Here is a description of what they have from their website along with contact information.

Grand Army of the Republic Post Records, 1879-1938

Records for individual Indiana posts, such as post files, descriptive books, minutes books, and post histories. Records frequently include personal data.

Email: arc@icpr.state.in.us

[Note: When sending an email to the Indiana State Archives, please be sure to include a very clear subject heading, i.e, Civil War, Soldier's Home, Hospital Records, Morton Telegrams, etc. Emails without a clear subject heading may be mistaken as spam or a potential virus and be filtered out automatically. If you have not received an initial response from the Indiana State Archives within a few weeks, please resubmit your request.]