Right under where the slider goes on the door you should be able to find the brand. On my 4 Winds, its a Dexter. So I googled Dexter RH screen door slider.
Perhaps you can take that route. If yours is a Dexter... here. Found it on Ebay.
4701-00 Dexter RH Screen Door Slider Clear for Series 5050 Radius Corner
Cheers
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