Thanks for the replies. I agree with all. Especially what Jasmine12 said about prolonging a pained animal's life for the pet-parent's ease of mind being selfish.
And I'm sorry, Zerafian. I didn't mean to conjure up bad memories for you by asking this hypothetical question.
To answer the question of where donor kidneys are procured, it's much the same as living-donor transplants in people. A cat, like a human, only needs one kidney to function so living donors can continue living after the donation. Often cats, perhaps from the same household, are tested for compatibility. In the case where transplants are done at research universities' veterinary hospitals, compatible research cats are sometimes used. (This brings up a whole other section of ethics as the use of live animals in research is an entirely other topic of conversation.) As far as I can tell, cat transplants are living-donor only, no cadaver organs are harvested.
As some of you know, I've been through both personally. Not with my cats but as a human patient. I've had three kidney transplants and my most recent period of 3x/wk hemodialysis treatments lasted from 1995 until 2012. And don't think there weren't many, many times especially during the last dialysis period when I didn't consider voluntarily discontinuing treatment and opting for, essentially, euthanasia.
As Calyx rightly pointed out, human renal treatments and those for cats should be considered two different topics entirely because of communication ability, patient understanding, levels of medical advancement, availability of insurance, etc.
But the fact remains, these are still options out there for the grieving pet parent who cannot bring themselves to let go and have the means to pay for them. Again, in most instances it's probably done more for the human than the animal and that is selfish, IMO.
Jeff with the girls, Penny & Nala
RIP Simon, my friend