I mix up my own solutions using either KCl or K2SO4. Using Chuck Gadd's calculator, mixing 76g of KCl in 1000ml of H2O creates a solution where 1ml raises 10g of water 1ppm. The amount of K2SO4 needed to create an equivalent solution is 85g/1000ml H2O.
The nice about mixing your own is that it's cheaper and you also know the the concentration of the solutions. With commercal ferts you dose based on the manufacturers recommendation, but rarely will that tell you how many ppm you're actually adding.