Search for Joseph Rosa, he wrote three books on Hickok as well as Age of the Gunfighter, and other titles about the Old West. He makes a reference to Hickok practising in one of the books and cites the source, which if I remember right was from the memors of an old Abilene resident.
Hardin, in his autobiography cites practising with his six-shooter, and in a fight in Kansas with some Mexican herders he say he had worn out his revolver practising and had to hold the cylinder with his off hand to shoot.