Please contact DCIN...all 3 of my diabetic kitties were adopted through DCIN they still help me with testing supplies and insulin for them, although ideally they would rather see that the cat stays in the home with the people they know and love. The ones that I have adopted had special circumstances for why their original owners couldn't take care of them.
Even if DCIN can only help with getting her into the vet's to confirm that it is diabetes and get you a script for insulin, we can help you do all the rest. Might first suggestion is just switch her diet, while you are waiting to find out for sure what is wrong and are getting in touch with DCIN. Putting her on a low carb/high protein canned diet is the very first place to start and will help to lower her bloodsugar. Then when or if you start her on insulin you will have a better baseline to work with.
With my own Maxwell that was all it took to get him diet controlled. He was 485 when he was diagnoised, took him off dry food, and started him on a low dose of insulin, 2 weeks later he was back to normal bloodsugar, gaining weight and acting like a kitten again. He just turned 13 years old and you wouldn't know he was 1) diabetic or 2) not one of my 5 year olds.
Venita is a fantastic lady and will help you all she can.
Mel, Maxwell, Musette, Autumn & The Fur Gang