Basically standard cavalry pants, many times with reinforcements in the seat/thighs. Many would be wearing the gray uniform shirt or civilian shirt, frequently in a checkered pattern. It was very hot so probably few if any uniform blouse/coats, they would be rolled across the saddle. Neckerchief of some sort. Hats would be variable, most would be broad brimmed, many civilian, though some would be the kepi style forage cap. While riding they might wear leather gauntlets, but once they started firing most probably took them off,as it was hard to load and fire (and extract jammed cartridges!) wearing the heavy gloves. A knife of some sort. No sabres. They boxed those up before leaving on campaign. They were considered too heavy and noisy and of little use, though after the fact some of Reno's men might have thought differently. The Hollywood image of nicely uniformed soldiers on the plains is just that, a Hollywood image.
Custer on that day was wearing buckskin pants, blue "bibbed" shirt and red neckerchief. His buckskin coat was rolled on his saddle and not worn. Wide brimmed hat of a light color, maybe gray.
Cheers
Jason