CapitalAquaponics buys food and will sell it by the kilo. Otherwise you're best off buying it from the hatchery which may do it in small quantities. Ideally you want to be moving your fish onto larger sized foods as they grow.
The issue with buying fish food direct is that you have to buy it in 20kg bags. The fish will grow fast and you will want to move them onto larger food as they grow, so you will be stuck with something like 15kg of unsuitable fish food. The vitamins tend to break down in them so they are not really suitable to keep for years at a time.