I got my magnets here:
http://www.kjmagnetics.com/products.asp?cat=168
I think they were the 1/2" dia 1/4" thick ones, (but mine weren't black, they must be out).
They made me buy 5 of them (minimum pricing or some bullshit), but it was still only about 7 bucks or so I think.
Bottom line is that 2 of them, 1 North side up and the other South side up, (glued to the biggest metal ring washer I could find at Home Depot, which is then glued to the fan using a hot plastic glue gun -- see this picture
http://www.brewiki.org/StirPlate), works just fine with the stir bars I got from Northern Brewer.