Very sorry to hear about your cat.
I had to put my dog down in January, she was 14. She had stopped eating for a few days and kept throwing up everywhere. I took her to the 24-hour vet hospital in Deerfoot Meadows (near Cosco) because I had no idea what was wrong with her. They kept her overnight and ran a bunch of tests, and the next day when I came in to see her they told me that she had a mass the size of a fist in her chest, many small tumors in her bladder, and that her liver was failing, among other things. They said they could try to treat some of the things, but basically it would just be prolonging the inevitable. It all happened so fast, and I basically had to decide to put her down that same day because it seemed like she was in so much pain. Anyways, I definitely recommend that place I went to, they were very nice there and took really good care of my dog while she was there. When I decided to put her down, they took care of everything ahead of time, I paid my bill and made the arrangements for cremation so that I wouldn't have to deal with it afterwards. They gave me a private room to spend some time with my dog before they put her down, and gave me lots of blankets and stuff so that she could be comfortable. They were good about not interrupting or rushing me, I was basically allowed as much time as I wanted with her before going through with it. When I was ready, the vet came in and explained everything, and was really nice about it. Watching my dog "fall asleep" for the last time was really hard and sad, but since she had been so sick I knew it was for the best. After it was done they gave me some more time, and then since everything was all paid up, I was able to leave when I was ready. A few weeks later they called to let me know the ashes were ready.
Anyways, it was definitely a really really hard thing to go through, but the clinic I went to tried to make it as good as they could. I'd recommend them for sure. Hope you get to spend some good quality time with your cat before you have to go through with everything.