I tie tons of C-Duns each year
To get good C-dun hair, you HAVE to look it over before you buy it. Coastal Deer hair can be really good, if it's the right piece. Look for a short coarse hair with short tips (the darker colored part right at the tip. The shorter the tip, the more flair you will get closer to the tip. The more coarse hair will not flair like a finer deer hair and is perfect for C-Duns.
Young Elk hair is also a good choice as is Texas Whitetail.
If you want whitetail hair to be good for C-Duns, it would have to be harvested really early in the year, the beginning of archery season before the winter coat sets in.
You can also call Chris Helm and let him know what you need. He will send you the perfect piece of hair every time.
Good Luck,