Check amazon.com, they have a pretty decent online plant store and every plant I have ordered from them has arrived alive. Similarily, check with your local plant or pet shops as they might be able to order one for you! They're normally very willing to order products you may not be able to get on your own.

I am not sure where you live, but I know that Aquarium Plants Factory have a large selection of aquarium plants at affordable prices. They also often have a lot of promotions going on.