Insulin for Leah

My cat Leah is almost 15 years old. She is the last pet my mom got me, and has been very important since my moms passing three years ago. I've noticed little changes in her, such as weight loss and increased thirst and water consumption. I didn't really see anything else wrong until this morning. I saw her running to and from the litter boxes trying to pee. I thought it might just be a uti and I called my vet. I was told to bring her in immediately. I dropped her off and was later called back. They told me they found glucose in her urine. I couldn't afford the $220 for the lab work to check her organs and confirm the diabetes. My vet did a glucose check and her blood sugar was over 400 where a normal range would be in the lower 200's. I'm basically in a pickle because rent is due and I can't afford the initial start up for her blood test, insulin, needles, prescription food and follow ups. If anyone can help, I'd appreciate it beyond words. My vet feels like she can live well with insulin. I can afford the upkeep when it's time to get more insulin and suuplies in 3 months. Thank you again!