starting comfrey is easy, if you have a good location. It needs sunshine! I started some in the shade and it did very little. I love to get starts from friends as it adds to the sentimentality of my garden.
If you don't want your comfrey to spread [as much] don't dig and disturb the roots, use only the leaves, or plant it in a container. It is an amazing plants with many healing properties. I love a cup of comfrey herb and peppermint tea in the evenings. Good Luck!
another place to buy the herb already dried and ready to use in ointments, etc., is Mountain Rose Herbs (mountainroseherbs.com) they have excellent organic herbs.