I made solutions of calcium nitrate and sodium monobasic phosphate from reagents at my laboratory (I'm a Neuroscientist). I tried using fertilizers and using the amounts printed on the labels, but the numbers I got back on those were not very accurate compared with the laboratory reagents. Not too surprising.
I think you can get fairly pure sodium or potassium nitrate at drug stores, but since they can be made into explosives, you might have to talk to the druggist a bit to get some. Sodium phosphate shouldn't be too hard to get as it is used as a buffer. You do have to take the formula weight into consideration because most of these are hydrated (meaning some H2O is also present in the crystals).
You might try your local University or High School Chemistry department if you'd like to mix some up.
By the way, I've found that the nitrate test kits change sensitivity once they are opened. They can go either way, either becoming more, or less sensitive or go one way at first, and then go the other way. The changes can be 10 ppm or more.