Mainly human with different skin tones, and they wear no clothing aside from harness and some jewelry; weapons are swords and rifles.
Native American models would probably serve well with some minor conversion, but without traditional head dress or mohawk-type hair styles. Fantasy minis would work for some, but they'd also need compatible rifle-armed figures.
The only native American 6mm I know of is from Irregular, and they have no pictures of these on their site (and, being from Irregular, I'm concerned about the "blob effect"), and I haven't seen any 6mm ranges that will serve well without lots of conversion work.
Black Hat makes an 18mm Barsoom range (Giant Tribal and Imperial Martians) which is pretty good for getting the general visual idea of the setting.