Insurance for senior cat.

My DD's cat has been living with us for the past 2 years. She has been paying all the vet bills and taking him for his booster has etc.
Recently she and her family moved away and they signed the cat over to us. I'm hoping someone can recommend a pet insurance company that will cover a cat aged about 15. He was a stray that DD adopted so we don't know his exact age, this is just an educated guess. Thanks.