When I was concerned about niacin the first week I had my ducklings (since all I could find was chick starter and couldn't locate a vitamin supplement or brewer's yeast; seemed everything duck related was disappearing from shelves at the time) I gave them a little bit of tuna mixed with brown rice every day. Both are natural sources of niacin. Non-vegetarian humans get most of their niacin from meat and fish, but it's also naturally found in whole grains and other foods as well, as you see from the link you posted. I'm actually writing a book on the chemistry of nutrition so I've done a fair bit of research on niacin and other vitamins, just...much more in relation to human diets, not ducks

I don't know the niacin content of bugs and worms and slugs but I would guess that is one primary way ducks get it in the wild/foraging since that would be their "meat". Also minnows and probably frogs. Another thing you might not think of is that some algaes have high niacin content, and ducks in the wild eat a lot of algae. Seaweed also has good niacin content, and I've tried to find if other aquatic plants do as well, but it can be hard to find information on vitamin content in "foods" not widely eaten by humans. But I would suspect so, since it is the aquatic birds that need the extra niacin, so I would guess they are getting it from foods land birds wouldn't eat as much, if at all, such as aquatic plants and algaes. Growing your own chlorella or spirulina for example would be one way to add niacin availability to your flock in place of purchasing commercial feed or vitamins
Edited to add: as I've stated before I'm just getting started with ducks, so anyone wanting to switch off commercial feed should do a lot of their own research first and not just run with what I've said. I'm still using commercial feed and will continue to until I'm confident of the quantities/ratios of different foods necessary to meet their nutritional needs