You are probably gonna have to look online somewhere. I am looking for a few for our deck and nothing I find in the stores is "approved" for top of step use!!!
And can I suggest these safety knobs...best thing ever invented!!!
They are better then the "normal" kind (the ones that have the rubber grippy thing you grasp to turn the knob) because my last two (yes even my 20 month old) can kind of "hang on" those gripper kind and the doors pop right open!!!