This post contains miscellaneous images and other material related to The Sixth Gun RPG. I'll add to it as more items and ideas jump out at me.
To start, here's a map of the United States during this time period (1887).
Next, here's how the American flag looked when there were thirty-eight states in the Union.
Continuing my picking through of Wikimedia's images, here's a sketch for a Hand of Glory.
Here are a couple of maps; one lists the sites of major battles in the ongoing war between the US Army and various Native American tribes; the other shows all of the rivers that feed into the mighty Mississippi.