Most of your large commercial size quantities come from vendors who sell to business' only. You can certainly shop at Sams or Costco for large bags of sugars and flours. The only other way is if you have a friend in the restaurant or bakery business that would be willing for you to add on to their order.
If you have a GFS store in your area, you can walk in and buy the commercial cake mixes from them, too. Some places have a will-call option, where you can place the order and then you have to go pick it up. Sam's also has a commercial customer representative who will help you and order you anything you need.
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