The big problem is that the quanties you are looking for are just "quantity" purchases and not "bulk" packaging. For example - a bulk bag of flour or sugar is going to be 50-pounds! Here is one source: Honeyville Grain, Inc. where you can get a 50# bag of cake flour and a 50# bag of sugar (shipping included) for $89. I do like their flat $4.49 per order shipping cost.
You might want to spend a little time Googeling the items you are looking for, but put the word BULK at the beginning, like "bulk cake flour", "bulk cake icing", etc.
When it comes to things like shortening - I would buy only the quantity that you will use with 3-6 months - the stuff can go rancid which can result in having to either throw out rotten tasting product or tossing unused shortening. Commercial/wholesale suppliers like Bunge Foods make several "shortening blends" that are geared to specific things, like baking. They are usually sold in about 35# boxes. You'll just need to find a distributor.
The problem with "buying in bulk" from a 50# barrel at some place like GNC, or my favorite food store Central Market, they are not always cheaper per pound than buying a bag off the shelf.
As others have suggested - break out the phone book and do a little research for local suppliers of bulk/wholesale food items. Or, join a Sam's Club/Costo and buy in smaller quanties.