Does anyone know of a source for these?

I occasinally snap the head off a screw and need something to back them out
of the wood without carving a huge hole out for pliers. I would imagine what
I need is something that's kind of a mirror image of the Craftsmen screw
removers... I need something with a concave cutting "dimple" that would fit
over the end of a screw shaft and bite into it, allowing me to back it out.