How do I get my parents to get me a horse?

I've ridin horses before and am getting lessons so I looked up on the computer "horses for sale" and I found this website called "Equine now" and there's a filly I saw and immediately fell In love with it but she's $1,500 and we dont have that money yet but besides the money, how do I convince them to let me have her because if I don't hurry, someone will buy her before I get too.