Costco for flour/sugar in bulk.
Otherwise, see if there's a restaurant supply store in your area that is open to the public... I do a lot of bulk shopping there (dried beans, rice, 1lb of yeast for 3$, even stuff like huge bags of buckwheat and masa...). The bags are always 20lbs+, though, so you have to be able to store it somewhere out of thee way! Still, the prices by weight tend to be about 1/2 to 1/6 of what the local grocery store charges.