Our petco has a rescue section (many of them returns from customers), but they also take in rabbits that aren't available for sale in the store, so they accept non-returns as well. They also work with the local humane society and have cats there for adoption. At my Petco at least, they charge you an adoption fee that is a donation to their rescue program, rather than profit for the store. However, in my experience, it's really variable how fair the adoption fee is. We got Bowtie and Tuxedo there as rescues (former biters) and they were only 10 dollars with a free plastic carrier thrown in. Then they had a female rat in a cage inadequate for even a hamster, and they were charging 25 dollars, and wouldn't allow you to take the rat without the cage--because, as an idiot employee said, she's used to the cage and needs to stay in it (right...).
There's a rat there in the rescue section that has a tumor the size of a golf ball. It's some sad stuff.