I buy a lot in bulk. Where I get it depends on what I'm buying. In general, check ebay. More specific would depend on what you are looking for.
The trick to doing it is to be prepared to do math. Know the price per pound it costs you now, what type you want, then start looking, comparing price per pound of the item PLUS shipping.
So 10 pounds of something at 3.00 a pound plus 17.00 shipping is 47.00 divided by 10 equals 4.70 a pound. How much are you paying locally for it? If it's less than that. keep looking.
I also buy cheaply from discount stores that sell items that the grocery stores send to auction for asst reasons. One of the
local stores has been getting a lot of restaurant supply foods in bulk. That's interesting to me. BUT! Don't buy anything you won't USE!! When in doubt, skip it.
I have been to Costco and Sam's, my math has me not buying a membership. They have big cans, at a great price for that product, but it's of brand name products. I don't buy brand name, generic is half the price. Cheaper brands in smaller cans maths out to cheaper on the products I was looking at.
Do your math! And check ebay, AFTER you know the price you are currently paying, and exactly what you want.