I found a friend who's had magnet rings; he pointed me to some info: http://hackaday.com/2012/01/02/give-yourself-a-sixth-sense-o... (follow the links; it links to "Holy Scrap" blog which links to a couple online magnetic-ring-sellers).

I also found a ring sizing guide: http://pages.ebay.com/buy/guides/ring-sizing-guide/