For dosing nitrate, I'd use a food grade sodium or potassium nitrate. I won't link them since they change rapidly, but Amazon sells some and Loudwolf sells some that can be obtained on ETSY. Calcium nitrate is likely also OK and is often available.
For dosing phosphate I'd use food grade sodium or potassium phosphate, and here there is substantial variability since you can have 1, 2 or 3 sodium or potassium ions per phosphate. All are fine if food grade, but the potency changes a bit. Same link issues.
For dosing these materials, this calculator can often help:
http://www.theplantedtank.co.uk/calculator.htm