Most of my pond plants I get from regular yards. Canna, Umbrella Plant, mint, Easter Lilly, Giant Reed, Mallow, sedge, Iris, Ruellia, Horsetail Reed are all plants I've gathered here in Phoenix from neighbors which I grow in water holding containers (mini ponds). Back in San Jose there were even more choices, like Astilbe which I haven't found here yet. Neighbors are surprised how much better these plants do when planted as marginals.
Often neighbors are happy for help dividing the plants. For Giant Reed I got a very nice lunch in addition to the plants in exchange for the help. I am lucky to live in a neighbor where gardening is very popular and can walk around perusing the inventory.
And my fav plant, Canna, I will only collect from neighbors. Virtually all sold today have a virus while many in people's backyards are still virus free. I still keep them separate until I'm sure they're virus free.