You most likely won't find it listed in the seed catalogs. There's been a lot of negative publicity about comfrey, and it's on the naughty plant list. IMO, it's just more of the same bla, bla, bla, from government and folks who want to keep us dependent on processed animal feed. I got mine from Coe's Comfrey. I doubt that it would be happy in a pot. The stuff has a massive root system, and mines minerals from 10' deep. Keeping it in a pot would be counterproductive to the benefit provided by the plant. Coe sells a Russian variety that is sterile.
Maybe if we all grow enough, they'll pay us to stop growing it. Isn't that the way our super intelligent government works? Pay the farmers to stop growing crops that are beneficial??? Perhaps I should check to see what other crops I might not plant this year. I could then retire from not growing stuff!
I mostly don't want it taking over since it spreads through root division. I run a soap company and it's great for lotions, salves, and soaps. I have room to grow in the ground but not enough for it to spread.