How can I persuade my parents to let me get another cat?

I currently have two cats but I went to the shelter and fell in love with a female cat (she's not spayed). I asked my parents but they said that it would stress out my other two cats way too much. Also, my dad doesn't really like cats...he's OK with them but still...I spent like 10 hrs. yesterday researching and making a book on how to introduce cats but I don't know how to start the conversation, etc. I love my cats to pieces and I make sure they always have food, water, clean litter, etc. and give them plenty of attention. I want this cat soooooo badly and I would pay for it and everything! I have over $2,000 saved up in my own bank acount. I want to get the cat before it is adopted (although it is not available for adoption yet, but will be in a week!). Don't bother answering my question if you are not serious. Is there anything I can do to prove that I'm responsible and to prove to my parents that I can introduce them properly?