I'm sure you could find a comfrey plant or root if you google it - various gardening/seed catalogs carry it. But if you know anyone who has some in their garden, they could just dig up and give you a chunk of root to start it - grows super fast, very large, and it's pretty, with slender stalks of purple flowers. It's nearly indestructable, you can just yank out a bunch of leaves from time to time, and my chickens (and my horses, too) just love it. Actually, my chickens eat about everything I grow, but I've noticed they are particularly fond of kale and chard, and these will give you a nice long growing season, providing good green stuff through most of the fall.