Many of the leading garden catalogs now carry some tropicals, and more and more local nurseries are doing so as well. Elephant ears and many of the more common exotic plants are widely available locally. Some lesser-known ones are, too, so it's definitely worth checking with your local retailer first.
For hard-to-find showstoppers, these websites deliver:
Logee's Plants for Home and Garden (logees.com 888-330-8038)
Almost Eden (almost edenplants.com 337-375-2114)
Plant Delights Nursery (plantdelights.com 919-772-4794)
California Carnivores (californiacarnivores.com 707-824-0433)
Longfield Gardens (longfield-gardens.com 855-534-2733)