Does anyone know if records survived for these prisons?

My g. g. grandfather, Harfield McCormick, was captured Jan 11, 1863 at Arkansas Post and admitted as a Prisoner of War to Gratiot St. Prison Hospital, St. Louis, Mo., Jan 12, 1863.

He was transferred to Hickory St. USA General Hospital, St. Louis, Feb. 12, 1863. He was discharged from that hospital March 14, 1863.

He was a surgeon, and served in the 6th La. Cavalry

If there are any records surviving from either hospital which may have any information about him (physical description, etc.), how might I go about ordering them?

Thank you for any information you might help with,
Barbara Langston