There are over a dozen MAJOR businesses on the internet, including my company, that sell nothing but plants. I am surprised nobody has mentioned them except John! If you want common every day plants, or if you can trust people on EBAY, then that is one source, but legitimate businesses that focus just on plants can offer a much larger variety including many plants that you just can not find anywhere else. Do a search on the internet for "aquarium plants", look at each WEB site carefully. Look at how indepth each WEB site is, and then make a desicion based on what you are looking for.
BTW, I will also add since you are in New Jersey, there are two stores in NJ that I sell plants to, including many of the rare and hard to find plants. Absolutely Fish and FishTown USA. Fishtown has just about every single plant I sell. Great selection, good prices, very helpful. Ask for Evan.
Maybe you better not go with them... :/