I can understand why pet stores can't sell dogs or cats because many of them are bred just to make money. I don't support buying one from a pet store. I suggest getting it from a friend or a shelter.
I got a cat last year. A friend of mine gave it to me because he is an idiot for not spaying his cat, I had to take one in because he could not take care of that many kittens. My parents were opposed with me taking it home. Me and my siblings fought to protect the kitty from being thrown out. Anyway, I can support this cat because I have a job. And taking care of a cat is very expensive, especially if it's a kitten. Food and cat litter has to be bought at least every month and paying for the vet is even more expensive. Plus a cat can be problematic if they don't get fixed.
If you can afford it and think you can handle taking care of a cat then go for it. Otherwise just get a goldfish.