The place with the most plants is usually Triangle Tropical Fish in Durham. They've got a new website, www.triangletropicalfishnc.com
that lists the new stuff they get in stock. Sometimes you find some pretty neat little gems there that you'd normally have to order online, like less common crypt and anubias species.
Raleigh has The Fish Room, but it's focus is mostly saltwater and the freshwater plant selection, while nice, is limited. Wake Forest has Aquarium Outfitters. Also a nice place, though I find the plants to be a little pricey.