The three diabetics I’ve cared for were all best controlled with a combination of diet and Levemir. Fancy Feast pate-style and no dry food; no need for “special” (read “way overpriced” food).
Check with Diabetic Cats in Need about the best ways to source Levemir or Lantus. I used to live in the Detroit area and just drove to Canada for it where it is over-the-counter and less expensive…