Yeah, it's a specialized "zoo food". It's expensive compared to feed store poultry feed. But not all feed stores have balanced waterfowl food, not all people go to feed stores, or have one near them. In some areas, they are a thing of the past. The fact you can get free shipping or $5...
If money is a problem, dry cat food, such as the Purina ones, have a lot of niacin in them. But I don't know if ducks absorb the nutrients in cat food like cats do. I just know I fed dry cat food, along with other grains, to dumped domestic ducklings in a park for years, and none developed leg...
If your ducks are less than six months, this is what would be a balanced feed for them:
https://www.chewy.com/mazuri-waterfowl-starter-duckling/dp/248788
For older ducks:
https://www.chewy.com/mazuri-waterfowl-maintenance-duck/dp/248786
Shipping is a flat $5, or free if you order at least...