I also purchased my Stir Bars from eBay. I received 5 of them in the lot that I bought. When I first played with my Stir Plate I did have the Bar I was trying keep flying off, even when running the Fan at a very slow speed. I thought that something was wrong with they way I had set everything...
Here is my $20.00 Stir Plate. It about an hour to build. I did my first starter on it this last week and just pitched the Yeast. It seems to worked great. I used a 10K ohm POT, not much control. If I was doing it again I would change to 100 ohm.