I guess this is more asking for resources and scholars to read/watch than it is a specific question, but AskReddit said my question may be better suited for this subreddit than theirs.
Long story short, I'm a writer, and I've decided one of my characters is going to be Native American, but that I'm going to create a fictional tribe as to avoid any misappropriation or misrepresentation of living cultures' customs and beliefs, and allow for more freedom in adapting their beliefs to fit the world they're in.
But, even if the tribe I'm making is fake, I need to do my research. I want to do this properly, and make this tribe as believable and respectful of existing tribes as possible, but I have no clue where to even begin. I don't have any indigenous friends or friends of friends, and I don't know a single reputable source for information on any individual tribe, let alone multiple, since what I create will likely be an amalgamation of many influences. Perhaps even branching off of preexisting tribes in some way in the distant past.
So, where are some good places to start my research? If there are any educational videos on these topics, those would be of interest as well. Tribes specifically native to Montana would be ideal, since that is where this character will be coming from.
First, since you are asking for resources specifically for a fiction writing project I'm going to first direct you to r/IndianCountry's wiki, specifically their essay on Is it appropriate for me to write about Native Americans? How can I do so respectfully?. You are not the first non-indigenous author to ask how to include an indigenous character, so I would start my journey with the resources and direction they graciously provided.
Second, as a fellow author who is not of indigenous descent, I strongly encourage you to examine the reasons you want to feature a Native American character, and the implicit bias you will need to reckon with to do justice to the character. Bluntly stated, we inherit the ghosts of five centuries of genocidal violence. Even with the best intentions, and desire to be respectful, unconscious biases will influence your writing. Learning the history will help, but after two decades reading and writing about Native American history I still make frequent mistakes.
Finally, here are some recommendations for books to get you started. You mentioned Montana, so I'll point you to a couple of general resources and then some for life on the northern Great Plains...
Colin Calloway One Vast Winter Count: The Native American West Before Lewis and Clark is the single best introduction to the deep history of the American West. I absolutely love this book.
Ostler's Surviving Genocide: Native Nations and the United States from the American Revolution to Bleeding Kansas will set the stage for understanding the origin and development of U.S. Indian policy, and how native nations adapted, responded, and resisted.
If this is a modern character something like Treuer's The Heartbeat of Wounded Knee: Native America from 1890 to the Present will bring the story to a modern setting, with the added bonus of reading history from an indigenous historian.
Elizabeth Fenn Encounters at the Heart of the World: A History of the Mandan People will give you a nice introduction to the Mandan who live in North Dakota, and whose villages served as a major trading hub for the Plains.
Pekka Hämäläinen Lakota America: A New History of Indigenous Power will likewise provide a good introduction to the Lakota.
The Cheyenne and Arapaho both currently have reservations in Montana, so A Misplaced Massacre: Struggling Over the Memory of Sand Creek by Ari Kelman will help you understand the deep history of violence, distrust, and how we as local communities, and a larger nation, try to reconcile with a genocidal past.
I'm less familiar with the western Montana nations, so I'll leave recommendations to those who know the area better. Please contact the indigenous nations you would like to draw information/culture/traits for this character, and see if they have resources they provide to aspiring authors.
This is the time to do “real” research. Skip the internet. Contact: The tribes themselves State office of minority affairs state office of Indian affairs county and local offices of Indigenous people local, county, and state libraries local elementary school teachers who do units on the area and local community local government to know what sorts of laws are particular to your area which were influenced by the indigenous community. Also, all of the above for the Canadian province just north of you for their First Nations resources. This is not something that you can look up online; this is something that requires legwork and talking to people who can share resources and information.