Does it snow a lot where you live? I have discovered that not all zones are created equal. If your zone 6 is under 5 feet of snow all winter, the ground is somewhat insulated. It rarely snows here so the soil surface is the air temp which could be 5 degrees. On average, the ground freezes 18 inches down so anything that can't take being frozen into a solid block for the winter does not survive.
I have never seen them for sale at any of the local nurseries but next time I go, I will ask.
Daisy