Well-Sweep Herb Farm had it listed in last year's catalog under "Symphytum: Comfrey, Common Purple; Russian Symphytum x uplandicum". Catalog number was SYM220X. Check with them for this years info.

Here is a link that might be useful: Well-Sweep Herb Farm

Comfrey does not need to go to seed in order to spread - it spreads very rapidly by sending up new plants from the roots .. or any severed portion of the root! These roots spread incredibly fast, far and deep. I would strongly recommend planting it as you would mint - in a pot sunk in the ground.

Come to think of it, the comfrey escaped through the holes in the bottom of the pot, which was a good 10" deep.