Pet Fly's size medium carrier is really big, and I think would probably fit your 15 lber. I agree you might have better luck with a stroller though! 15 lbs is a lot to carry - I have a very hard time with my 11 lb girl.
Welcome To Pet Fly's - Trendy, Edgy, Airline approved pet carriers for traveling in style, comfort and safety on board a plane, train, automobile or just hanging out with you. - the medium or super-size would be most likely to fit your girl. You can find Pet Flys for sale on several websites, so just because something is sold out here doesn't mean they might not have it on another website.
Just Pet Strollers - Free shipping on pet strollers from Pet Gear, Kittywalk, Happy Trails, Petzip, Burley and more - great prices on strollers here. We have an AT3 All Terrain and it easily fits both of my girls.