I am making a stirplate myself. I was very frustrated at first with all the issues of throwing the magnets. I am using a 15v laptop supply, and before anyone yells at me for using too much voltage there's a reason.
Since my divorce, I don't have stockpiles of old computers laying around, so I had to acqure a fan. All the new fans are silly optimized for sound, so unless you lay out $10 or more for a fan, you aren't going to get one with enough strength to turn two hd magnets. I did have a 80mm that worked out well, but it didn't have the balls to spin a couple of N55s I had from K&J magnetics.
Two things to note that I discovered that I haven't read anywhere yet.
1) if there is metal between the stirplate and the magnets, you don't have much of a chance using a fan to turn the magnets. This is because when you move a magnetic field through metal, it generates a magnetic field in opposition to the direction you move your magnet. This opposing force is offset through a 90 degree motion of flux and a tangential 90 degree motion of current. (think x, y, z in three demensions) in other words, a metal plate robs you of motor force, if you need a metal plate (for example a heating plate) under your starter then you need a MUCH stronger motor.
2) North and South - remember that these magnets from harddrives and from KJ magnetics are axially magnetized, that means North and south are TOP and BOTTOM - NOT on the ends like a traditional bar magnet... You are going to throw a lot of stir bars using one magnet, or if you have two magnets with the same two poles facing up. If you use one North-up and one South-up magnet, those stir bars will survive a hurricane.
3) (I know I said two) I have seen this before, but I like reiterating it. We aren't making a vita-mix blender... it just has to swirl the water a tiny bit to be successful.
Prost,
Antje