I found a set of 50 magnets ready for putting business cards on at the dollar store (United Dollar).  I bought  5 sets and had 250 magnetic business cards.  I feel more comfortable leaving these on a car or two at the grocery store and other places that I have permission to or it seems okay to leave business cards.  I've even given them to family and close friends and aked them to put one on their refrigerator and they always seem to just say okay.