[MethodImpl(MethodImplOptions.InternalCall), WrapperlessIcall]
         internal extern AnimationState GetStateAtIndex(int index);

using UnityEngine;

public static class AnimationExtension
{
     public static AnimationState GetStateAtIdx(this Animation animation, int num)
     {
         if(animation = null) return null;
         int i = 0;
         foreach (AnimationState state in animation)
         {
             if (i == num) return state;
             i++;
         }
         return null;  
     }
}

AnimationState state = animation.GetStateAtIdx(i);