I don't have a diabetic cat, and I'm sure there are probably people here who can provide you with some more detailed information and helpful links, but I'm sort of peripherally familiar with the condition because I have a condition (reactive hypoglycemia) that involves some of the same processes and dietary concerns.
Basically what diabetes is, in brief, is the body's inability to properly regulate blood sugar levels. When you consume foods that contain sugar (or foods that are broken down into sugars, like carbohydrates) your body produces insulin as a reaction, to keep those sugar levels from going too high. In a diabetic person or cat, the body has either become insulin-resistant, or is unable to even produce enough insulin to lower blood sugar levels, thus they remain abnormally high, causing the various health problems attributed to diabetes.
Therefore the most important thing in a diabetic diet is carefully controlling sugar intake. In humans this is a bit more complex, but in cats it's thankfully much simpler due to the fact that they're obligate carnivores. They don't even need the foods that tend to spike blood sugar levels.
What will be spiking blood sugar levels in cat food is carbohydrates. Carbohydrates are a food humans need for energy, but cats don't have nearly the same carbohydrate requirement. Foods containing too many carbohydrates, especially grains (corn is probably the worst, as it has a very high glycemic index) can cause and aggravate diabetes.
And ideal food for a diabetic cat is one that is 100% grain-free, with a fairly low carbohydrate content. This means grain-free canned food (dry foods, even grain-free ones, contain more carbs than canned foods) or raw food.
Some grain free canned foods are:
-Wellness (grain free flavors are marked with a yellow triangle)
-Merrick Before Grain
-Blue Buffalo Wilderness
Commercial raw brands:
*You can also find a plethora of information on homemade raw diets in the raw subforum on this site.
These are not complete lists, just some brands I know of off the top of my head to be good foods.
I hope that helps! The good news is, with a proper diet, some diabetic cats don't even need medication, as their symptoms an be controlled simply by diet alone. However these are options you should obviously discuss with your vet. Only your vet can tell you whether or not insulin or other medications are required for your cat's condition.