Are you sure you want duckweed? It's an exclusively floating plant, will not root to any surface and won't cling to anything except your hand or nets anytime you dip them into a tank, or it will dry onto the sides and under the frame when the water level drops, leaving you to scrape it away.

It's a great way to provide shade and a hiding place for tiny surface swimming fry, and I use it in some breeder tanks, but otherwise, it's a real mess in a show tank.

You can tie java moss to logs or screens though. For java moss, check out Aquabid. There are usually a number of listings for it.

Personally, I find Water Sprite to be a great aquatic plant. It can be rooted or floated. I used it for baby livebearers to hide in, as well as medium for breedding bubblenest breeders.

For my livebearers, my favorite is Naja. It grows like mad, looks nice, is easy to clean if any algae starts on it, and it's simple to thin out.

But I think that darkruler is looking to use some type of moss to decorate the back wall of a show tank, and for that, they are probably right in trying java moss. It's cheap, grows in most any conditions and is easy to tie onto a mesh background.