Are you hoping to contaminate a large body of water or just get some for a little pond or aquarium? If the former, its highly illegal; and if the latter, then it will grow on its own in a year or so and you will have as much as you want.

I have found it on Craig's list in our area. You might try there or if there is a pond store near by. You can get it at pet stores but it is very expensive. I can't keep it in my bigger pond because the fish eat it. They also like the fairy moss, and other little floaters but I grow more of those for them in my big pot ponds. I give them a big handful and by morning they have devoured it. If you have to go the expensive route you might want to set up a small pot pond to grow some in.