MagnaCard magnets from OfficeMax or Staples. you have to place on each of your cards yourself but they are thicker and stronger than any of the other magnets out there...that i have seen. being stronger makes them more practical for refrigerator use.
i often check eBay...last time i bought a lot of 10 cases for $10/per case w/100 magnets per case...will last me a few years.