Lima minor is said to be the fastest growing vascular plant there is, with doubling rates as low as 4hrs with ideal conditions. It does get everywhere, tho, b/c it's so small. If you can find Lima major, it grows almost as easily, but is easier to keep out of places you don't want it b/c it's significantly bigger. In Canada, you can get either from many aquarium hobbyists, so I imagine it'd be similar in the US... know anyone with an aquarium that has live plants growing? There's a good chance they'll have some duckweed. If not, check the local pet store. If they sell live aquarium plants, there's a good chance they'll have duckweed. Lima minor is by far the more common. You may have to do a little leg work to find Lima major.