Most garden centres sell them in the season. This can get expensive, though, as you can never have enough! Also some spray their plants with insecticides to keep them looking good. These are unsuitable to use as food-plants.
You can however buy seeds from the Trust. Plant them NOW (whatever the time of year) and grow them so you have lots of food for your Monarchs in the Spring. Seeds are $5 a packet. http://www.monarch.org.nz/monarch/items-for-sale/seeds/
Once your plant is well established you can save your own seeds and have plants growing forever.
