any plant store shoudl carry something along the lines of it.. but i would probably try for a hydro store, and get "calmax" or something specific for hydro. not sure if it makes a difference... but.. why not.. its only $10-12 a liter.
however, in a pinch, epsom salts work for magnesium (like $10 for a 2 pound bag) at walmart, or any drug store.
and if you are worried about calcium , i have never done it, but apparently, you can wash egg shells with h202, then drop them in your res, and over time, the slightly acidic nutrient solution will disolve the egg shell, and give you some calcium + other goodies.
however, in a pinch, epsom salts work for magnesium (like $10 for a 2 pound bag) at walmart, or any drug store.
and if you are worried about calcium , i have never done it, but apparently, you can wash egg shells with h202, then drop them in your res, and over time, the slightly acidic nutrient solution will disolve the egg shell, and give you some calcium + other goodies.