Today I went out and found some of the artichoke plants hunched over and dropping mature seeds on the ground. I cut the dried heads and put them into some white paper bags. I gathered the bag up around the stem and started whacking the thing against the back of a patio chair. I could hear the seeds falling out into the bottom of the bag. When I poured the contents of the bag onto some row cover the seeds came tumbling out along with various insects and spiders. I picked out all the seeds and put them in an envelope.
Once I finish collecting seeds from all the artichokes, I'll count the seeds and determine if I have enough to offer through Seed Savers Exchange annual yearbook, which members of SSE are given each year. SSE suggests having enough seed for 10 requests of 25 seeds each, for a plant like artichokes. I could offer as "limited quantity" if I don't have enough for 10.The recommended number of seeds varies by plant type. For example Maize (corn) should have 200 seeds for the minimum offer since corn needs lots of genetic diversity to avoid inbreeding. Suzanne Ashworth's book Seed to Seed is an excellent source of information on growing plants for saving seed.
I obtained the seeds for my Imperial Star artichokes from the Natural Gardening Company in Petaluma, CA. They offer certified organic seeds and plants. I also got my organic strawberry plants from them.
