I just placed an order to Bulkreefsupply. You can do the pre-portioned bags for a very small premium, and it still works out to magnitudes cheaper than buying any of the store stuff. Just empty a bag into a gallon container, top off with RO, and you're on your way.
There are also great calculators all over the place online for calculating a dosing schedule.
All of the 2 part are generally the same stuff with generally the same outcomes, so why not do the cheapest? A gallon will last you years in a nano. With my current consumption of ~5mL per day, I'll still get a number of years out of a gallon, and it was like $3/part/gallon