Phosphate Chemical Test Kits (CTK) are available for basic water quality testing. There are two options for a phosphate CTK. One is for basic color cube with 5 points of resolution while the other option is for the Checker Disc that has 50.
Multiparameter CTKs are available for boiler and cooling tower water treatment. Both kits include other key parameters.
For educational use there are the Hanna Backpack Labs. Both of the Water Quality and Marine Science Backpack Labs that include a phosphate test kit along with a teacher’s guide that discusses the importance of the parameter.