Trying to convince my mom to get anoher puppy?

I have a 9-year-old shih tzu that I got when she was a puppy (9 years ago), and I used to train her in agility. She is long since retired from jumping due to arthritis and back pain, and I have been hinting to my mom that I want another shih tzu puppy. I am 14 and my parents are divorced, my shih tzu (Skittles) travels between my parents' houses with me, and my dad has no other pets. My dad would let me get one if the puppy could travel with me like Skittles, but my mom is skeptical because she already has her dog (mutt from SPCA), my cat, and my guinea pig (that i would be willing to find another home for). How can i convince my mom to get me another little jumping prodigy?