I intend to use a semi-mythical setting - I'm no expert on the mid-west, living in sunny Canada as I do, so I'm not going to be able to present an exact locale. There'll be plains and grasslands, desert, forests... I'll be watching some spaghetti westerns and flipping through a book or two to help me out. Certainly Sioux territory will be represented (Dakotas, Nebraska, Iowa, Missouri, Kansas). Crow and Blackfoot areas (Wyoming, Montana) as well. But hey, I just want to set up some cool "cowboys and Indians" battles, not remake history.
For now, I'm going to work on my own, but if I need help, I'll definitely ask for it.