Well, why didn't Barney just go to the store and buy his own damn Fruity Pebbles? Him and Fred work at the same place so you know they make approximately the same wages. And if cost were really an issue, he could get the generic brand for half price. But I doubt cost was an issue since Barney was always buying those costumes and props to trick Fred. Nevermind Fred being a complete asshole, but why couldn't Barney just be a man and buy his own? That'd make a lot more sense that going to all those elaborate means just to trick Fred into giving him some.
How hard would it be for him to just chisel some onto the grocery list and Betty could pick up a few boxes for him?