There are a few supplements that some people have found beneficial in studies.
These include cinnamon, EPA, among others. You can experiment and see if they make any difference.
However, the most important thing to manage diabetes is not supplements, it is what you eat every day. Many people find eating low carb diet (avoiding sugars, starches, grains, sweet fruits) along with exercise and meds (if necessary) are the best approach.
Many of the supplements on the internet are IMHO just marketing ploys to make money for someone and to take your money. Start with the basics of your diet and lifestyle.
You are right in doubting supplements as an alternative: healthy eating is the way to go!
I am taking Bitter Melon, Cinnamon, and Gymnema Sylvestre, and have noticed a small decrease in my A1c. I am doing this under my doctor's care. You can ask your doctor if you could try this. You will have to check glucose levels frequently to make sure you aren't going too low, and adjust your diabetes medications to match.
I take vitamin C 2 tablets daily am & pm 1000mg each. It helps. The best is diet and exercise.
Excerpt from Diabetes Self Management - Magnesium: The Forgotten Healer...
"The NIH says, “Magnesium is needed for more than 300 biochemical reactions in the body… [It] is involved in energy metabolism and protein synthesis.” And according to our own Amy Campbell, “Results from three very large studies indicate that people who consume a diet rich in magnesium have a lower risk of getting Type 2 diabetes.”
People with diabetes are more likely than those without to be low in magnesium. According to an article on About.com, “Elevated blood glucose levels increase the loss of magnesium in the urine, which in turn lowers blood levels of magnesium.” So getting enough magnesium is especially important in diabetes.
In spite of these benefits, medical authorities rarely recommend magnesium. That’s why I call it the forgotten mineral."