Our last four cats have come from a Petsmart program - a local volunteer rescue organization works with them.
Previous to that, I found a wonderful cat by looking in the want-ads. This was before the Internet, so I don’t know how individuals with a litter of kittens advertise them now; but in those days, people would advertise kittens and give them away for free.
Bulletin boards in vet offices are another good idea - lots of people who have had to euthanize their cats will be looking for another one.
Sorry, the company here that works with volunteer rescue and where we’ve gotten our cats is Petco, not Petsmart.