Where can i get only one mallard duckling?

Please do not do this. A duck will imprint on humans in general, they will be virtually impossible to rehome. It will not be able to be released into the wild, it is not easy to raise a single healthy duck even if you are a mother duck. Your school should be aware of these problems - there are hundreds of ducks in animal shelters or just dumped to fend for themselves. Ask if you could get extra credir for doing the research and NOT having a live duck!! I am sure your teacher will understand that this is a very BAD idea. Whereabouts are you? Perhaps you could volunteer at a rescue centre instead?