Go to a lake that is at a higher altitude, preferably in a forest type area, especially if it is a bird-sanctuary swamp. Duckweed are little floating plants, a couple of pea-green spongy leaves about 1/8" diameter with a little thread of a root, maybe 1/2' long. They reproduce so well as to form floating blankets that block the sun from the weeds below.
In your aquarium they might be a pest, getting into the filter, or your fish might love to snack on them.
If you get them, or anything from the wild, rinse well and quarantine first, to make sure you didn't bring home something you didn't want.