It's Atlanta, man! Any farm pond or swampy ditch near you should be loaded with more kinds of duckweed than you knew existed. Lemna, Wolffia, Spirodela, maybe even Wolffiella. You'll probably scoop up some other floaters too: Riccia, Riciocarpus, Azolla, perhaps Limnobium. It's all good!
You can probably find cheap water hyacinth at any place that carries pond supplies unless it is illegal in your state, in which case you're just out of luck. Check these same stores for duckweed if you're uncomfortable wild-collecting it. They'll often give it to you free.