Karen, yes anyone can order from them and they take orders throughout the year. They get a lot of orders just after the list the new plants for the year, which happened a couple of weeks ago. They even waited with shipping till we had a dip in temperatures here.
Here is the link to the catalog entry page:
http://www.huntington.org/bota...
You can also order from previous years, but they request you inquire about availability before ordering as they have sold out of many of the plants from previous years and they do not have the man power to keep the website updated with that. You can get the spotted mangaves from Plant Delights Nursery, but they are more expensive there and shipping from PDN is pretty expensive.
