Try finding a koi pond supply store.
I have a pond for my goose installed with a pump and a filter for koi ponds. Kois are supposedly almost as messy as geese. Mine are an Oase Aquamax 4000 pump and an Oase Filtoclear 3000 filter. This has worked fine for more than three years now. The filter is very easy to clean. I supply with using a fine meshed net once a week or so to rake out the worst poop and leaves, especially from the bottom and the surface of the pond. Only problem is I can't use the filter during the winter; it will freeze. But the pump alone keeps the water ice free most of the winter.
My pond holds about 400 gallons of water, and both pump and filter are a bit undersized for this. Geese poop a lot in the water! I share my goose with a friend, so she's only with me half the time. If she were staying with me all the time, I'd have to get a much more effective system. Right now I'm guessing the water quality is fine - a small colony of newts have moved in!
A couple of years ago I had to empty the pond - it had been invaded by lovesick frogs who continuously tried to mate with my goose's feet. She was so scared she wouldn't even go near the pond and she kept hissing at it. I relocated 36 frogs to other ponds in the neighborhood.
I'm not sure adding turtles and goldfish is a good idea. Both will contribute to polluting the water. Turtles are omnivores and may attack or bite your geese. Even the small ones have sharp "beaks" - I once had a 2" pet turtle bite a tiny chunk out of my finger. If the turtles thrive, they'll grow bigger and potentially more harmful fast. If they don't thrive, they'll die, rot, and pollute the water.
I tried planting Typha and Juncus last year, but both were gnawed down to stubs within a few weeks, and only the Typha has survived. Geese love to play with plants, even the ones they don't eat. Don't expect a goose pond to be a romantic decorative pond. I don't think lily pads will survive for long.