Yes a veterinarian is the only way. See a veterinarian and they will make sure its comfortable for you and the cat.
Christown Animal Hospital in Phoenix is our vet and has very reasonable prices
It's probably easier to go to your vet vs here. It's about $113 here sir, but there is other low cost services in line for about 50 dollars. https://www.maricopa.gov/5424/End-of-Life-Services
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