Don't forget to look at the superb works of artists Karl Bodmer and George Catlin if you want detailed, authentic depictions of Native people in color, before the advent of photography. Their combined work is invaluable as reference, painted right from the source.
Curtis' photographs and the site referenced, while still interesting, depict the reservation era and its material culture, which is much later and vastly different than the early 18th to middle 19th century, so often the theme of most sculpts.