hypoallergenic dog food

What ingredients should be on a hypoallergenic dog food?
I have a two and a half year old golden retriever and I noticed even when she is on medication as the advantage flea bites yet I know it's not hot spots because they have red spots, but looking at your skin I realized small red bumps and very dry skin, when I took her to the vet suggested omega 3 fatty acids puffs brought me salmon oil and salmon, but she was still itching and his coat is very full, not as a thinning of gold should be. I changed the food science diet for large breed dog for real the golden retriever, but I think she has a food allergy from corn and wheat are two ingredients in your real dog. I know of one to three food allergies commonly affect goldens so my question is does anyone know the ingredients in a food is hypoallergenic and any good food for dogs with allergies. I work at PetSmart so any food that could be called expensive or I can not find and I get a discount so the price does not matter, thanks.
It all depends what your dog is allergic. You may have to try a few different foods before finding one that works for her. free grain foods is your best bet. I would say trying Orijen, Wellness, like the forest, grain free food Acana. If none of the work, Acana Lamb and Apple is a great food. A source of protein (lamb) and only a source of carbohydrates (oats). This is what I feed my golden retriever who has food sensitivities. http://www.championpetfoods.com/acana/show-product.php?formulation=la
James Wellbeloved Natural Healthy Hypoallergenic Dog Food