This is where I get my rare earth magnets for holding stuff on the airplane and for the occasional ignition. They have all sizes and they are cheap. The last time I ordered I got a bunch of different sizes for less that $20.
http://www.magnet4less.com/
I used to be able to get them at Radio Shack but the last time I went to my local RS they had been discontinued. May still be some in older Radio Shacks.