What is a good commercial food for diabetic dogs?

I have an eight year old diabetic Shih-Tzu on insulin twice a day. My vet advised what ingrediants should be in her food and recommended Science Diet WD that he sells out of his office. When I found better alternatives after researching dog foods online he made excuses for sticking to Science Diet. He would not give me a prescription to buy it where my wife could get it at a discount. I have to travel nearly an hour one way to get to his office and an hour back for a bag of prescription dog food. He is doing the same with the insulin. Insisting I buy it from his office. I am convinced he is not looking out for the best interest of my pet but rather for himself. I have switched to another vet nearby but suspect I may run into the same problem. My research tells me that Science Diet is dog junk food. My dog has done well eating Blue Free Range Free Lamb and Brown Rice for the past year and a half. It seems to have what is requireds for a diabetic dog. Has anyone else had this problem? I would not recommend that anyone blindly trust the recommendation of a vet. They are in it for the money.