The missing roller in this picture is manufactured by Fasco in Canada. They are usually available in nylon and steel ball bearing varieties.
I like the steel ball bearing style best. Installation is very easy, you just push the wide flat spring into the open pocket in the screen door and click the roller assembly onto the 2 prongs protruding in the top of the pocket.
If the prongs are broken off, you would have to replace the corner inside the screen door frame as well. I have seen these rollers in Home Depot but most good glass shops should carry them.
