Cheapest, easiest way to get comfrey is
root cuttings. You can plant them anytime of the year assuming the soil can be worked, basically as long as the ground isn't frozen. The heat won't stop them, so if you find any feel free to plant them.
I bought crown cuttings ( the "main" section of the
roots) from Coe's Comfrey. They also
sell root cuttings, which are cheaper. I received a few freebie root cuttings when I purchased, stuck them in the ground, and went about my day. I thought they died (I planted them in November) them one day in spring they just basically exploded out the ground lol.
Anyone
local with plants can easily get you some root cuttings. Simply pull the plant up, cut some 2-3 inch lengths out of roots about the diameter of a pencil, and you've got root cuttings. Plant them about six inches deep, with plenty of manure or
compost, and lay the cutting down horizontally. Root cuttings take will need most of the first year to develop a root system before you can really harvest the leaves, but they'll survive and they'll thrive. I plan to do this this fall from the established crowns I planted last year. If I didn't have use for them, and didn't live on the other side of the country I would send you some.
EDIT: I'd stay away from seeds. Most cultivated comfrey is a sterile variety. Planting a seeded variety can lead to wayyyy more comfrey plants than you want.