I'm not trying to sound flip, but you probably want to decide and/or obtain your new big bird before trying to source their food. First, you might want to know what food/foods your new bird eats, if you get the wrong food in advance, you'll probably be stuck with the food.....
You'll probably need to also know what your new bird's eating capacity is, to help you figure the capacities you'll be needing, unless you have a large, unused freezer you can package and store excess food in, by excess I'm taking about no longer than 90 days.....if not properly packaged for freezing, foods develop freezer-burn, which imparts a funny taste to perishable foods and you could end up with a lot of extra food your new feathered friend will not eat.....
I'll let some others suggest online vendors with good prices and their 'toos' & macaw's favorite foods, as I don't order my foods online.
Good luck.....