If there is a Banfield, The Pet Hospital, in your area I would recommend them. My daughter works in administration here in Portland and so we do take our dog to one of their clinics. I have been impressed with their warmth and friendliness on a professional level. They can quote fees for you over the phone. I've been irritated that the cost for the care of animals is as expensive as that for people. However, the Banfield clinic is less expensive than the clinic I was using before them.
Also, here in Portland, we have a clinic named Companion.. I don't know if they're a chain. But if they are they are the least expensive that I've found. My brother takes his dogs there and says that they're about 1/3 less than usual. We also have a non-profit.
I suggest that you look in the yellow pages to see if you can find a non-profit clinic or one that advertises lower fees. You may have to call a few. IF you ask any of the offices they can probably tell you of a non-profit. Or call your county animal control office or the Humane Society. They will be able to give you information.
I am sorry that your cat is so ill. I have seen 2 cats who acted in a similar way to your cat and it was because of their age. I put down one of my cats, who was old and had cancer, when my daughter was around age 10. My regular vet was so cool and unsympathetic that I didn't have her put down at their office. I was fortunate to find a vet, recommended by a friend, who allowed my daughter to be present. He gave us a tour of the facilities and was so very kind and supportive as he gave the cat the injection. They wrapped her in a towel and put her in a box so that we could go home and bury her.
I hope that you find such a vet for your cat. It's painful to lose a pet even when they've lived a good life.