I have seen them in LA, but they are not called Jerusalem Artichokes. They are
called "Sun Chokes."
I probably saw them in Ralphs or Vons, where I usually shop. But I don't see
them all the time.
If you do decide to grown them, dig a big hole and put a container in it and
plant the chokes in the container. Otherwise they will take over your yard. My
dad used an old tin wash tub. If you like the sun chokes you find, you can
probably plant a few of them and they should grow into plants.