Oklahoma Historical Society
Encyclopedia of Oklahoma History and Culture: Arapaho, Southern
A short history of the Arapaho. It explains how they came to be split into their various divisions, the main ones being the Northern and the Southern Arapaho.
Wikimedia
Wikipedia: Wind River Indian Reservation
Peruse this brief summary of the Wind River Reservation, Wyoming, home to Eastern Shoshone and Northern Arapaho Indians.
Legends of America
Legends of America: Arapaho Great Buffalo Hunters of the Plains
A very good overview of Arapaho history. It explains how the Arapaho split into the Northern and Southern Arapaho tribes and their relationships with the Cheyenne and the Sioux.