When I think about it, though, if I had to recommend an acid for adjusting pH in a swimming pool, citric acid wouldn't have a lot to recommend it. Usually you'd want to avoid adding microbe food to water that you want to keep around for a while, and citric acid certainly has organic carbon that can provide calories. Also, if it's a cement pool, you'd want to avoid adding stuff that might attack it, which is why they say to keep up the calcium in the water in such a pool, and citrate will dissolve calcium salts such as limestone. The only advantages I can see to it are that it's a solid (but then, so are bisulfates) and that it's not too strong an acid (weaker than bisulfates) so you don't have to worry about the kids getting into it.
However, it did seem from the hits that came up that there might be some pool supply sources that might sell it anyway. And if it's a grade for adding to swimming pool water, it should be safe for bathtub water too, although possibly not as pure as what you'd get for pickling and other food use.