April/May is the time, just as they start growing in earnest.
Blocking 14 is the one to get if poss. Its seeds are infertile and so don't spread everywhere like all the other types.
Blocking 14 was developed by HDRA (Henry Doubleday Research Association) now called Garden Organic, base in Ryton, Coventry.
Their plants are : (£1.80 p&p)
COMFREY PLANTS Bocking 14 Pack of 5 £6.25
COMFREY PLANTS Bocking 14 Pack of 10 £9.75
Comfrey root cuttings are also usually available on Ebay in April/May, cheap, yes, but who knows what your actually buying ?
Also in garden centres, you will be able buy seeds (T&M) and 2" plug plants of the standard varieties, again around Easter time.