I just bought Daisy a harness like that!  I got mine at PetCo on clearance.  It fits pretty well - we haven't been outside with it yet, though.  
 
I, too have a "designer" dog - I have a chorkie.  She acts ALL chi and no yorkie, though!  She is very sweet and I wouldn't trade her for anything in the world.  Muts need love, too!!   
Welcome!  
