What diameter of the hole? I think you will have a better chance of finding a screw plug of a certain diameter/height than Stanley door screw plug, especially now that Stanley is no longer in the door business. Masonite (
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) had acquired their door business. In theory you could call them at 1.800.663.DOOR (3667) and inquire about the right size, but I would just measure one and get replacements from any number of hardware stores, including online. Amazon's special small parts store comes to mind:
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or just about anyone that comes up on a search for "plastic screw plugs"