The most important question: What is your climate?
If you are in the tropics, you can throw some floating plants in. Water Lettuce, Water Hyacinths for example, but they are tropical and cannot survive cold weather. I buy them every Spring, but have to throw them out come fall.
Do you have shallow shelves built into your pond to sit plants on?
If not, there are other ways, like floating planters or planting them in the ground and have them flowing into the water (Creeping Jenny).
There are a lot of online pond plant sellers, but the shipping can be expensive.
Note that a lot of plants that are called "pond plants" are actually regular plants that can be bought at home improvement centers and local garden centers for substantially less money. Iris, lizard tail, Creeping Jenny, papyrus, umbrella plant, Marsh Marigold, Parrot's Feather, Anacharis and many others. These are all winter hardy and grow larger every year. (Some Parrot's Feather are winter hardy, some not).
I have Water Forget-me-not, Aquatic Mint that also return every year.
I have not had luck with lillies, my fish destroy the branches. Lillies are nice and come in tropical or hardy, but they don't help much as far as improving water quality.