First I want to say that you're awesome! This kitty chose you, and you're not letting the little bugger down! Makes me all teary and stuff...
In addition, I would just expand on what Janice said, a lot of small local vets are willing to work with people who rescue stray animals, even going so far as offering spay and neuters for super cheap! Also, there is this program called Care Credit, they offer instant online approval, and the credit card is specifically for health care for family members...including pets. It will even give you a list of local providers (in your case: vets) if you want to check into it. I know I got my Care Credit card and was able to get 18 months of deferred interest...Just another thought!
Hugs!