Hach sells them.
I use this kit myself and does work well at the lower end of the phosphate range. The sell re-fills much cheaper, so once you by the complete kit, you need only get the regeants at much more econcomical prices.
This is the Hach PO-19 low end phosphate kit:
http://www.hach.com/phosphorus-orthophosphate-reactive-test-kit-model-po-19/product?id=7640214958
Phosphorus, Orthophosphate (reactive) Test Kit, Model PO-19
Gallery Product #: 224800 USD Price: $84.79 Available
Ascorbic acid. Range (mg/L): 0-1, Smallest Increment- 0.02. Range (mg/L): 0-5, Smallest Increment- 0.1. Range (mg/L): 0-50, Smallest Increment- 1. Approx. Number of Tests: 100. Case Style: 12x7.25x6 inches Blue Polypropylene Carrying Case.
"¢Results in less than 5 minutes for clean water samples
"¢Utilizes simple color disc comparator for accurate measurements
"¢Model PO-19A contains filtration apparatus for turbid samples