What do you mean by large quantity? There's bags that are by the kilogram, others by the gram or ounce. For online food, you can buy it from nearly any pet store franchise such as Petland and Petsmart, or you can consider locally owned companies. My personal favourite is Things for Wings based out of southern BC. Another good online store is Parrotdise Perch, based out of Calgary or Mississauga. It will be very difficult to find a quality food online that is "large quantity" for anything under $30.
In terms of pellets, Zupreem Natural, Harrison's, Tropican, Roudybush, Goldenfeast, or TOPs are all good pellet brands. Some are better than others in terms of quality, but ultimately your cockatiel will decide that for you [emoji4]