This may not answer your question, but this is from my data base.
The 6th Michigan Volunteer Cavalry Regiment was mustered into federal service in Grand Rapids Michigan. They were raised by Representative Francis William Kellogg, under the authority of the Secretary of War. George Gray became the colonel. The 6th Michigan Cavalry were reportedly issued blue shell jackets, reinforced mounted pants, a kepi and boots.
i The horses were all purchased in Michigan; Company A received bays horses, Company B brown horses, Company C gray horses and so on. In February 1863 the 6th Cavalry were armed with Colt revolvers and sabers. They were issued sabers, Spencer repeating rifles, and single-shot breech-loading Burnside carbines. The company commander Captain James H. Kidd's first uniform was ordered from a tailor and was of union blue with brass buttons, bright yellow facings and the shoulder straps of a captain of Cavalry.
ii
i Stowe, Mark S., Company B 6th Michigan Cavalry 2nd Ed., self-published, Grand Rapids, 2002, p. 3. Mr. Stowe does not note were he gets this information and the accuracy of his disruption can not ve verified.
ii One of Custer's Wolverines the Civil War Letters of Brevet Brigadier General James, H. Kidd, 6th Michigan Cavalry, Edited by Eric J. Wittenberg, The Kent State University Press, Kent Ohio & London, 2000, p. 11.
Officers probably purchased tailor made uniforms for their first uniforms, perhaps/probably a Michigan tailor. After that they would have purchased new uniforms from a tailor near where they were stationed. So I guess a tailor made frock coat with two rows of buttons. Probably U.S. staff buttons, but perhaps he would have purchased Michigan Staff Buttons. Private purchased sack coats were also popular. These would be tailored made and the tailor would have provided anything the officer wanted. As a major he likely had a frock coat and some form of campaign coat or jacket. Much of what you want might depend on if you want to show him in garrison or winter camp or on campaign. Many Michigan officers opted for tailor made loose fitting, long, five button sack coats.