Hi Allyson,
I know you can buy seeds, now that being said, I am not sure if a greenhouse would carry them or you might have to buy online. I did find a holistic site that sells the comfrey leaves $30 a pound. So you might want to check out some of those sites as well as a healthfood store.
Thank you for your kind words, let me do a bit more research and I will keep you posted if I find some direct sources.