I got mine from a smaller fish store. I have seen some at petsmart and other larger chains.
There are also places to order them online. Some of them will even set up packages for you depending on the particulars for your tank.
It's worth doing a bit of research and shopping with a list. Some stores sell plants that are not fully aquatic and will slowly die away once you plant them in your tank.
Though, most of the plants listed here are fairly common and readily available.