If There are quite a few companies that make cavalry figures that would work in a diorama of Quantrill's Raiders. By 1863, quite few of the "Boys of the Bush" were wearing captured Union uniforms, so one could use Union Cavalry figures with pistols, (sans sword and kepi of course, and go with a slouch hatted figures). A few confederate cavalry could be thrown into the mix, and they wouldn't have to be painted gray, as civilian-colored pants or coats would be fine. On the confederate figures, a few should be painted in their guerrilla shirts (red or brown), and if you wanted to include a guide on bearer, Although I don't think a guide on was used much, but if so, it would have to be the black flag with a white "Q" in the canton. A few of the troopers could be carrying carbines, but very few, as pistols were the preferred weapon.
As a general rule, Quantrill's Guerrillas before 1863 would be wearing civilian clothes and guerrilla shirts: 1863 and after, a lot of blue uniforms with civilian and guerrilla shirts mixed in.