Or, you can buy pure:https://en.wikipedia.org/wiki/Trisodium_phosphate
in the paint section of your local Home Depot for peanuts and add it to your detergent(it's marketed as a paint stripper, but it's safe, all detergents used to have it) and add it yourself. You can buy commercial grade dishwasher detergents that are allowed to contain it due to an exemption, but this way you can add it to the washing machine too, for which there are no exemptions.