For those of you who use trisodium phosphate for cleaning your filter cartridges, where do you usually purchase it?
HD/Lowe's usually carries it. Paint stores, too. There's apparently a TSP substitute (green) that's being put out there and I've heard it's not nearly the same but I have only heard that....never used it.
It is not legal to sell TSP in some states, so you may not be able to find it. Don't use the fake substitute TSP that does not contain phosphates, it is no where near as good. If you really can't find TSP, you can use automatic dishwasher detergent instead. It is almost as good as TSP.