Google: Huntington Digital Library; Photographs; search for Sioux or any other item you wish. Under the Sioux search will turn up many photos we are all familiar with. The gems are the glass plate photos of Fort Totten, the agency, St. Michael School, etc.
The Ft. Totten Indian items are a photo of a Sun Dance, begging dance (looks like an agency distribution) A hay (Grass Dance) dance, and a photo of a Wakan Wacipi (This is probably the only known photo of the Ceremony- pictured is the Akicita Tipi on one end and the Pejuta Wakan Tipi on the other end, with the enclosure fence (representing the reed bay the Water Spirit travels in) between the two tents.
There are also two photos of the Dakota teaching a Ft. Berthold group the Grass Dance (entitled Scalp Dance). Ten Photos of interest.