Help me afford vet bills and cremation for my cat

This morning I had to take my baby, my 15-going-on-16 year old baby cat Doofus, to the urgent care vet because he was SCREAMING every time he tried to poop, and the vet said upon initial examination that he felt like a water balloon.

Hours of crying later, I got a call saying that on the plus side, it doesn't seem like it's an issue with his lungs or his heart, but on the negative side, he's got some dark chunky fluid filling his abdomen and they're not sure why. It might be that the cyst on his pancreas that a previous vet had to drain has refilled or burst, or it might be that his intestines are leaking into his body. Either way, my options are to either put him down or find a specialist to do surgery on him, and the quote for how much that'll cost is close to $10,000.

I've already had to pay $1500 for the diagnostic testing, and had to request a credit increase to make that fit on my credit card. I can't afford the surgery, but I don't want to give up on the sweetest, most loving cat I've ever known.

Please, please help me try to save my baby. I can't offer anyone anything in return beyond my deepest gratitude, but I need help.