Lizzy, I hate to sound like a real monster, but you can't save them all. It is abuse plain and simple, but things will not change any time soon, because massive chain stores like PetCo and PetSmart make far too much profit to go out of business on account of several dying animals here and there. Protesting isn't a good idea either, as it might end as a night in your county jail. Letters that you write may never make it to corporate, and if they do, will be sorted into the rubbish as soon as they arrive. In the rare case that they do make it to a CEO, he will agree to change things but nothing will actually happen. I don't mean to sound ironic or heartless, but that's the way the world is.
So - be good to the ones you have, love them and spoil them silly, spend some quality time with them each day, and remember that you saved them from ending up dead or worse in a tiny PetCo cup.