Water hyacinth and water lettuce are both good floating plants. The pond needs to get some sun for these. I would look at garden centres that stock pond supplies and fish. Some aquarium plant distributors (online) sometimes carry pond plants.
If you want plants that are rooted in the substrate, or potted, you can use iris, some grasses. A stem plant like hornwort or Anacharis (Egeria densa) are others that can be left floating.
Byron.