So I am thinking of developing a persona of a poor Cherokee farmer from the Indian Territory who gets involved in the conflict, maybe for the CSA. Maybe as an older private serving under Stand Watie.
I'm not sure a battleshirt is what your looking for, Confederate Cherokee dress ran from citizen cloths any white man would wear, to drab (whites and browns) Confederate uniforms from Texas, to Union uniforms, and an odd ball fabric jacket particular to the Five Civilized Tribes, and dang near everything in-between and photographic evidence and first hand accounts don't mention battleshirts too much, if at all. Heck there are accounts of Stand Watie's troops making off with a shipment of black dinner jackets/tailcoats (accounts are conflicting on the type of formal attire) in 1864. For example:
This image has been identified as one Pleasant Porter who fought for the Confederacy, and was later a Creek Chief. He wears an button up "overshirt" (going with that term) trimmed at the top of the breast pocket and vertical stripes that most likely goes all the way around the shirt, ( a particular fashion among Natives, sometimes very colorful). Its worn over a standard shirt with a cravat, and a greatcoat over all of it. (I'm hesitant to call the greatcoat a Union one, and it may be, but it doesn't look quit right, it could have been one sent from home or Confederate imported in Texas, I'm personally not sure.)
Another Creek, (possible wartime image) Danial McIntosh, wears what could be Union trousers, an officers belt, a regular white shirt with what looks like a standard citizen sack coat, probably out of white linen or some other material, maybe even jean, its hard for me to tell as its so bright, and clean, it may have been new, or some other explanation, it just doesn't show much field wear.
And of course Stand Watie himself. Here he wears what looks like a Union sack coat to many, but I think it looks like a privately tailored black or other dark colored sack coat to me.
The key thing most people tend to forget about the Indians who fought for both sides, is they were not backward savages, in fact they were much more, there's a reason why they were called the "Five Civilized Tribes". Before the war, a program was in place to see them educated in good schools in the Indian Territory, and many were sent to white collages in Arkansas on their Tribal Council's dime, so the argument can be made, many Indians in the Indian Nations were better educated than many a white farmboy from elsewhere in the South, and while their were some fashions pecular to them, such as the vertical striped button up overshirts, many of them dressed the same as any white man, sometimes better.
Personally I would suggest not going for a standard battleshirt, if one could locate the proper fabric one could have the oddball striped button up shirt made, but I've yet to locate anything similar enough. So that goes to option two, standard citizen cloths, such as a citizen's sack coat or even frock coat, with regular period citizen's trousers. On other things, such as arms, I would say pre-war pistols for cavalry or no pistols at all, (I've come across accounts of Colt Dragoons, and Colt Navy's), and for a long arm, I would recommend either a Union issued gun in the Nations, (Austrian M1849 "Kammerbushe's" M1842 Springfields, etc.) or a simple civilian squirrel rifle, flintlock and percussion throughout the War, (the common type were Southern "poor boys" none of those cheap hawkin knockoffs).
For good reading and learning of the war firsthand in the Indian Territory, I suggest the memoir/biography "
A Creek Warrior for the Confederacy" by George Grayson, a Creek who was there. I know I reference the Creek a lot, but they photos of them are more prevalent, and they dressed near identical to and served with Watie's Cherokees.