Distilling your own can be much cheaper and convenient, but as with Dave you need to be careful of the NOx gases. Also depending on what type of nitrate you use and the stoichometry you need to be careful it doesn't foam up and cross over or get into the condenser of your distillation apparatus which can cause the foam(a combination of nitric acid and the bisulphate salt) to harden and clog the condenser. If that happens and you walked away and didn't get to it in time it can cause the whole thing to overpressurize and explode. I would recommend starting out only making 100-200ml at first until you get used to the reaction. I would also recommend that you calculate the amount of water needed to form the azeotrope as we really have no need for anything higher and it is just a hassle to store at higher concentrations.