There are a lot of different brands of food that are limited ingredient. Natural Balance makes several different varieties of LI food which I have used in the past.
LID Limited Ingredient Diets Dry Dog Formulas - Natural Balance Pet Foods
Pebbles did really well for a long time on the NB Sweet Potato and Venison until the changed their recipe and started using a different form of SP. I switched to the Venison formula and she still was scratching. So I asked my vet if we could try the Royal Canin Hydrolized Protien RX food.
Hypoallergenic HP Small Breed Dog Food | Veterinary Exclusive
She has been on this food for several years and I recently switched all three because Bogey was having anal gland issues that the vet thought might be allergy related. Everyone seems to be doing well on the RC food.