Check the trader section. I saw some for sale here. And to stop your fish from eating its roots I've seen other members here describe a hula hoop and netting type of floating basket that keeps the water hyacinth restrained in one part of your pond and the fish from getting at its roots. If you do a thread search you might be able to find that thread.
Oh yeah, and the members here on the forum buy, trade, and receive plants in the mail all the time.