Retailers like Tractor Supply Co. or Fleet Farm are excellent for buying in bulk, often stocking 20lb to 40lb quantities.
You can find individual high-energy suet cakes at Dollar General . where to buy suet for birds
Stores like Chewy stock a wide variety of flavors, such as insect, peanut, and berry blends from brands like Pine Tree Farms and Kaytee. Local & Specialty Options Retailers like Tractor Supply Co