Pets and Allergies

I don't know why it surprised me so much when I found out that two of my dogs have allergies.  For some reason, it never really occurred to me that animals (like humans) could be allergic to a number of things.  I started noticing it with Doc (my now 3 year old Pit Bull) when he was a puppy.  It started out as a small rash on his inner thigh but then continued to spread.  Of course, I took him to the vet thinking it was ringworm or something terrible like that but, no, he tested negative for everything.  (Thinking back on it now, I wish it had been ringworm....that, at least, has an easy fix)  The vet suggested it was allergies, either to a certain type of food, to an external source, or to some sort of bug.  I could almost immediately rule out the external allergen because he developed the rash in all seasons and we have all hardwood floors.  The vet wanted to put us on that specialty Science Diet food...which I would have gone with in a heartbeat had it not been fifty dollars for a 15 lb bag.  When I adopted Doc, I was working part-time, going to school full time, living on my own, and had another dog and two cats..it simply was not in my budget.  I quickly started experimenting with more natural foods trying to eliminate either corn, wheat, or soy.  I found Fromm Family Foods which were great!  We tried the Duck and Sweet Potato formula at first, which completely stopped his itching and red skin.  We later switched to the Gold formula because it was cheaper but would occasionally buy the Duck and Sweet Potato for them!  However, Fromm recently botched their 40lb bag and now only sells their 30lb bag.  Since we have three big dogs, it's too difficult to buy our dog food in 30lb increments...the extra 10 lbs really helps. 

So now we're searching for an answer to Doc's allergy problems.  Our older dog, Cheeks (an eight year old Cattle Dog/Pit mix) also has allergies which cause him to have awful ear infections and lick his paws.  I don't know if anyone here has a dog with allergies but what food do you use?  Does anyone have any suggestions for me on something I could do?