3 links have been added on this idea.
  1. make your own using

    2 matching ball bearings about 12 – 15mm diameter.
    1 3mm machine screw.
    1 Biro pen, makes a great blow pipe.
    A concave mirror to run it on, a flat surface will do but it will work better on a concave surface.
    Superglue, this is optional but is a good idea to help lock the threads in place.