I actually found Vital Brewers yeast at SPAR yesterday best Niacin supplement its about R60 here but its because they are robbing is blind, mix about a teaspoon in their drinking water. My water bowl for them is a 1.5L ice cream tub, they can get very messy.
I have goslings but it is the same for ducklings, they need a ton of greens make sure they get it on a daily basis.
This is the best greens for them:
Spinach (small amounts not too much, I usually give my goslings some once a week)
I normally pick the herbs from my garden in the morning and mix it in their drinking water they LOVE it. If you can, give them access to a lawn they need to eat grass on a daily basis.
FYI if the brewers yeast is in capsule form open the capsule, I got them in tablet form I crush it into a fine powder before I mix it in.