I am going to have quite a good supply of comfrey root cuttings available next spring. It is the Bocking 14 cultivar which produces sterile seeds. The easiest way to order it is from the Seedwise website:
I have written about comfrey and how to grow it on the website. I can also answer any of your questions here on the permies forum. By the way, it you are not familiar with Seedwise, you should check it out! It is a relatively new website that allows you to purchase organic, non-gmo seeds, bulbs and root cuttings directly from the farmer. Lots of fun!