It is sold as fertilizer in 50 pound bags in agricultural areas. If you are near any sort of farming go ask some of the farm supply places.
I mix my own, but not by the cup!
It takes so little that I am measuring it by the fraction of a teaspoon to handle 700 gallons worth of tanks.
Get your initial supply from www.aquariumfertilizer.com
and see how it goes.
Your recipe looks unbalanced. Did you work it out to suit some special problem with your water supply?
That is a lot of magnesium. Plants need magnesium and calcium in the right ratio. Not a good idea to add just magnesium unless it is to balance another source of calcium.
Plants need over a dozen elements to thrive. Some are quite easy to supply.
Hydrogen and oxygen, for example.
Some are needed in such a small amount that they are sold in a blend called Trace or Micros or something similar.