Be sure to get poison hemlock, not water hemlock. Water hemlock is a miserable, painful death.
For a great deal of detail on how poison hemlock was used medicinally, consult The Old Vegetable Neurotics, by John Harley, from the 1860s. You can find the entire thing on archive.org. Roughly a quarter of the book, the first quarter, is on poison hemlock. Harley was a medical doctor who took great care to try to figure out which parts of each plant contained the desired active principles, how to to extract the necessary compounds, even going to the lengths of experimenting on himself (although he would start with a mouse and work his way up).
One of the advantages of these older medical texts is that the methods tend to be less complex than current science, not as much equipment is required, much of the source materials are acquired from nature rather than Sigma Aldrich, and so forth. In short, the techniques are more reachable for someone who doesn't work in a chemistry lab.
My guess is that you'll have to plant seeds somewhere out of the way, in the right season, and from there wait for the harvest, then extract the coniine.