Some feed stores carry poultry vitamins and such - I got mine when I ordered my ducklings, from Meyer Hatchery. It's called Vitamins and Electrolytes "Plus." It's not the only brand available, though. If you ask for poultry vitamins they should know what you mean.
Sometimes people on the forum have used Pedialyte (I cannot remember if they use non flavored or which particular kind) for the eletrolytes. Some use a few tablespoons of plain yogurt - the kind with live cultures - in a cup of water for the probiotics. For niacin, you can get the capsules and dissolve the contents in the water - 100 to 150 mg of niacin per gallon of water.
Another thought: If the duck has eaten something toxic, you may want to give it charcoal (NOT the briquettes you cook with). You can find that at a drugstore. It absorbs toxins. The thing to remember is that it also absorbs niacin at least to some degree, so it's a balancing act. If the niacin makes the duck better, it's probably a deficiency and so the charcoal would not be much of a help. But if it's toxicity, then the charcoal would be more help than the niacin. If the duck is in the brooder all the time and you know it couldn't be that it's eaten something bad, then the niacin deficiency is more likely.
In any case, the vitamin mix will help the duck be stronger.