Jason: Thats anacharis, or otherwise known as Elodea. Quite common, but in most petstores it arrives in poor shape and dies soon after being put into a tank(plus, it doesn't do very well in low light situations).
If you can find a pond person who removes it from their koi pond by the bucketload, or otherwise a planted tank aquarist...then they will be able to give you healthy starter plants-and you should be able to keep them providing your conditions are good enough.
But I would reccomend java fern and java moss(perhaps anubias or cryptocrynes if you wish) over anacharis...they are much hardier.