See the Thistle Guide for more information on thistle production.
If you are new to this, you might try asking in a large public channel (like E! or your regional chat) if there are any silkworm farm owners whose tanks are asking for a simple type of thistle. It is common for farmers to make public their farms' requirements so that many thistle growers may benefit; the grower typically supplies the thistle and is returned raw silk at a rate of 1:1.
Alternately, you can always look at the Thistle Requests page to see if any farmers have listed open calls for particular thistle types, though this page is not always very current.