These we buy in bulk whenever they're on sale or when we can buy them from bulk bins.
Spices - both sweet and savory
Grains and Legumes - lentils, quinoa, rice, oats, and barley
Canned Tomatoes - whole and diced
Chicken and/or Vegetable Stock
Flour - though better to buy flour in smaller quantities if you don't bake frequently since it goes rancid after a while.
These are supplies we use a lot of or that we use infrequently but consistently. While we're always working to reduce our waste, buying these in bulk saves money and packaging of the product itself.
Dish Soap - we buy large jugs and refill soap pumps
Resealable Plastic Bags
Parchment Paper - we buy the longest roll we can find
Aluminum Foil - again, we buy the longest roll we can find
Plastic Wrap - ditto
One word to the wise: make sure you have enough space to store bulk items before buying! Most of these food items are fine un-refrigerated and can go in the back of a closet if you run out of pantry space.
What other foods or kitchen supplies do you buy in bulk?