I've really only gotten things from Petsmart and Petco. They normally have some nice beginner plants. After my first purchases, I learned to NOT buy tubed plants, only get stuff from the tanks, and even then be wary or at least informed!
I haven't bought anything online yet, but if I can get the bigger sorority tank moving forward, I would imagine I would have to, to get enough stuff to fill it nicely. So *proceeds to stalk this thread*