You can get a lot of emergency supplies at Costco, as well as stock up on a lot of food items. You save a lot of money buying some things, but you need to watch what you buy...note that I only mention Costco, but this is generic information and pretty much applies to any warehouse store.
You can get flour in 50 pound bags for about $12. Given that five pounds at the grocery store can run you anywhere from 30 - 50 cents a pound on sale, this is a considerable savings.
Where you run into issues is with large jars or cans of this and that. If, for example, you don't use a lot of Crisco, over time it gets kinda gnarly and gets sort of a translucent pale brown tint. Costco's 6 lb. containers of Crisco don't make sense unless you regularly use the stuff. You're better off buying it in smaller containers at your regular grocery store so you can leave them sealed until you need to start using them. Some things will spoil unless refrigerated, and since the point of prepping is to have things on hand in emergencies and emergencies often involve a lack of electricity resulting in a lack of refrigeration, avoid the humongous container of mayo or pickled asparagus.
Here's a list of some of what Costco carries that might be worth investing in for preps; - watch your prices; grocery stores sometimes have sales that beat the pants off the prices at Costco:
- Vitamins, within reason - they do need rotating, but are a lot cheaper than at the grocery or drug store
- Canned fruit and veg
- Flour, rice and pinto beans
- Toilet paper
- Cleaning supplies
- Paper plates, napkins, and paper towels
- Fresh fruit, if you can/dehydrate/freeze it (other than for immediate use)
- Parmesan cheese - doesn't have to be refrigerated, lasts a long time
- Pasta - spaghetti in several pound packages
- Spices and bouillon (although my local Cash N' Carry store has a much wider selection)
- Maple syrup
- Honey (although it's advisable to put it in smaller, glass containers in case it crystalizes)
- Bacon bits
- Dried fruit
Different Costcos carry different things depending on their local clientele; one store I've been to in the area carries big jars of ghee (clarified butter, doesn't need refrigeration), while my local one doesn't; mine carries pickled herring, another less than 10 miles away doesn't.
I find that my membership is well worth it but you should check it out for yourself before signing up. You pay about $3.75 a month to shop there at current rates, which is pretty reasonable if you shop there regularly.