There's some on yoox, ebay, Neimans, Saks, in addition to Prada stores of course. There was a purseblog entry about this, ha ha, two years ago
http://www.purseblog.com/accessories/prada-handbag-robot-charm/
and there was a link to a thread in the fashion spot forum, here's some pictures
the stiletto is my favorite one that i want to get