Hmmmmm,
My understanding is that brewer's yeast and nutritional yeast are not the same thing, but I don't know all the details.
There are a number of places from which you can order plain old niacin capsules. I don't think the no-flush is a good idea, from what I have read here, and it seems to me that it is a timed release formulation, which is not what you want, either.
By the way, if you can only find 500 mg capsules of regular niacin, you would just need to add that to five gallons of water, or divide the powder in half and add it to two and a half gallons of water. Easy for me to say! Sometimes storing that much water is far from practical. Forgive me if it seems hare-brained.
So, if you order things online, there are a number of places from which to obtain either the brewer's yeast or the niacin.
Hope that helps!