What if we created one? Maybe using a Tee Pee and or an eagle feather? Not something that would fly today but for the TL-191 CSA...I think that an emblem for the Indian Territory is going to be something that is originally made in TL-191.
I found battle flags and emblems belonging to different tribes, but none that was considered a universal symbol for all of the indigenous groups in the CSA; not even for the Indian Territory when it was part of the USA.
Examples that I found:
Choctaw Battle Flag
Cherokee Battle Flag
Website that gives you all of the Seals of the Five Civilized Tribes: https://fivecivilizedtribes.cherokee.org/Five-Tribes