I have the same issue, and it seems that the Louis Garneau helmets fit me great... especially the Carve. I'm not sure what type of helmet you are looking for, but that's a great fitting helmet. I'm a size 50, just to give you an idea.
I will give you an air of caution here. Many helmet companies make their helmets to a smaller size, but they all share a common shell size. For people with smaller heads, that means quite often the helmet can be adjusted to fit but the shell tends to move... which can cause many head/neck issues if one should fall. So, please in your search for helmets, please make sure that the helmet is snug ESPECIALLY the shell.