Oh, you may also want to look into some different supplements geared towards kidney/diabetes. Vetri-Science has a line of products. I give my current dog one called Renal Essentials. She had high bun levels, but they are in the normal range with these supplements.
You may also try the Only Natural Pet website. They contain a lot of different natural products. Pet Remedies also has homeopathic treatments for many different ailments.
I would still recommend a vet, because not controlling the blood Sugar levels can cause fast forming cataracts / blindness, kidney problems, and much else. However, supplementing with natural products is a good idea. Chromium is recommended for people with diabetes, but I'm not sure about cats.