The Green Z28 Blueprint linked is where I'd be looking, or even save a few bucks and find one with the 3.8L (96-02).
V6 Mustang convertibles are super common around here, and that $15K budget opens up a lot of options....Check the Toronto area listings and if there's something that catches your eye I am sure one of us in the area would be happy to check it out on your behalf.
Even 2010/11 Camaro Convertibles, while less common, are around sometimes for under $15K....not sure they're better than the Mustang, but they're a different option to consider at least.