Sometimes large home improvement stores or box stores will have them in their garden section, but your best bet is a nursery or garden center. If they don't have some in stock, they'll usually be happy to order them for you. You can also find them online:
www.mycarnivore.com/VenusFlyTrap
www.bugbitingplants.com/
www.californiacarnivores.com › Plant Catalog
www.mycarnivore.com/VenusFlyTrap
www.bugbitingplants.com/
www.californiacarnivores.com › Plant Catalog