That's the easiest thing. I found mine under a car at the place I work. There was a dog in the house and it was kind of a gamble, but it all turned out well.
I understand. From my experience, though, all it takes is a couple of days and your mom will be the cat's best friend! In any case, I hope that a cat will choose you soon!
Regardless of your religion's rules, buying or selling a pet is absurd. I mean a pet is like a friend, or companion. You wouldn't buy a friend, correct?
Now go find yourself a cat, but before that please make sure that you are ready for the responsibilities and expenses that come with owning a pet. They need food, healthcare and love, like humans.